US20080292516A1 - Method and apparatus for local fluorine and nitrogen trifluoride production - Google Patents

Abstract

The present invention relates to a process for production, shipment, and treatment of a NH₄F(HF)_x feedstock for local production of fluorine and NF₃ for semiconductor chamber cleaning without the need for storage of large quantities of dangerous feeds and intermediate products.

Description

• This application claims priority from U.S. Provisional application Ser. No. 60/561,180 filed Apr. 9, 2004.
FIELD OF THE INVENTION
• The present invention relates to a method for local production of fluorine (F₂) and nitrogen trifluoride (NF₃) semiconductor processing gases, such as chamber cleaning gases for large-scale semiconductor production facilities. More specifically, this invention relates to a method for remote preparation of a NH₄F(HF)_x solution that can be safely shipped to a F₂ and NF₃ production facility and for the conversion of the NH₄F(HF)_x solution to the F₂ and NF₃ products.
BACKGROUND OF THE INVENTION
• Semiconductor products are generally produced via batch processing steps that use gases to deposit or selectively etch semiconductor layers on substrates within a vacuum chamber. Most of the chemical by-products and unused reagents from these deposition and etch processes are exhausted from the chamber by a vacuum pump. However, some residue unavoidably deposits on the chamber walls and must be removed periodically in order to maintain product quality. Usually this residue is removed with gas mixtures containing some fluorine-containing cleaning gas, such as NF₃, SF₆, C₂F₆, or CF₄, which is usually diluted with argon or helium.
• Unfortunately, SF₆, NF₃, C₂F₆, and CF₄ have very high global warming potentials, i.e. respectively about 23,900, 10,090, 9,200, 6,500 times CO₂ on a weight average basis over a 100 year time-frame, respectively. While some fluorine containing cleaning gases have much lower global warming potentials, F₂ and ClF₃ for example, these cleaning gases are very toxic, highly reactive, and difficult to handle safely. These problems are exacerbated by the more recent trend to use semiconductor production techniques for the production of larger and larger flat panel displays that require a significant increase in the quantity of chamber cleaning gas. In particular, there is a significant increase in the associated environmental and safety issues. Moreover, because flat panel displays have much lower product prices per unit area than computer central processing or memory module units, non-productive cleaning time and the cleaning gas cost represent an increasing share of the total flat panel display cost. Therefore, there is a need in the art to ameliorate environmental concerns while maintaining safety and process efficiency.
• NF₃ is the most common chamber cleaning gas and is typically produced by the reaction of fluorine with a NH₄F(HF)_x salt, such as by the following reaction:
• 
  3F₂+NH₄F(HF)_x→NF₃+(4+x)HF.
• The reaction may be carried out in an electrolytic cell (as shown in U.S. Pat. No. 3,235,474) or in a separate reactor (as shown in U.S. Pat. No. 4,091,081). Alternatively, NF₃ production from urea and fluorine has been proposed (as shown in U.S. Pat. No. 6,821,496) using the following key step:
• 
  2CO(NH₂)₂+3F₂→NF₃+NH₂CONHCONH₂+3HF.
• All these ammonia-based NF₃ production processes use half of the fluorine feed to produce NF₃ and the other half to produce HF. Therefore, the direct use of fluorine as a chamber cleaning gas would be much more efficient than NF₃.
• Although F₂ is a more efficient and theoretically lower cost chamber cleaning gas than NF₃, elemental fluorine has generally not been used because of cylinder shipping and handling safety concerns. On-site fluorine production, via electrolysis of hydrogen fluoride (as described in US Published Patent Application 2003/0098038), has been suggested as an approach to eliminate the fluorine cylinder handling problems, as well as to decrease global warming emissions, and increase the fluorine use efficiency. However, on-site fluorine production faces two significant challenges.
• First, the quantity of the fluorine product that can be safely stored is severely limited by fluorine's high reactivity and toxicity. As a result, significant fluorine plant excess capacity is required to meet the highly variable cleaning gas flow rate requirements of a typical semiconductor production facility. In addition, the fluorine plant must be designed to minimize the probability that a fluorine plant outage and a disruption in semiconductor production. The risk of an outage and the very high opportunity cost for semiconductor plant outages economically justifies a separate back-up cleaning gas supply capability, usually NF₃. Therefore, the commercial need for a highly reliable chamber cleaning gas feed system and the highly toxic and reactive nature of fluorine generally requires an oversized and more expensive fluorine production facility as well as a back-up NF₃ supply system. In such a case, the theoretical cost savings can not be realized.
• Second, the hydrogen fluoride feed necessary for fluorine production is also highly toxic and volatile. Therefore, the large hydrogen fluorine feed inventories required, especially for flat panel display plants, pose a significant health risk that must be mitigated. For this reason, large-scale fluorine production facilities are usually located in relatively sparsely populated areas with a large buffer land area around the production facility. However, large-area display production facilities are often located in areas with high population densities and land prices. Therefore, there remains a need for a flexible fluorine and nitrogen trifluoride production and supply capability that avoids large inventories of toxic and volatile feeds and products.
SUMMARY OF INVENTION
• The present invention overcomes the disadvantages noted above by providing a method for remote preparation of a NH₄F(HF)_x solution that may be safely shipped to a F₂—NF₃ production facility and for converting the shipped NH₄F(HF)_x solution to a NH₄F(HF)_x feed appropriate for NF₃ production and to a HF feed appropriate for F₂ production.
BRIEF DESCRIPTION OF DRAWINGS
• FIG. 1 is a plot of the logarithm of the hydrogen fluoride vapor pressure as, a function of temperature with parameters of NH₄F(HF)_x melt acidity x value.
• FIG. 2 is a block diagram of a F₂ and NF₃ production facility.
• FIG. 3 is a simplified process flow diagram for a method according to the present invention to convert the NH₄F(HF)_x solution for shipment to appropriate NF₃ plant NH₄F(HF)_x feed and F₂ plant HP feed.
DETAILED DESCRIPTION OF THE INVENTION
• In accordance with the present invention an NH₄(HF)_x solution is produced by the reaction of anhydrous HF and ammonia. The anhydrous HF feed should be appropriate for fluorine production. Moisture is the most problematic BF impurity and should be less than 10 ppm. The ammonia feed should also have a low moisture content as well as a low and hydrocarbon content, each less than 10 ppm. The NH₄F(HF)_x salt solution is produced by the reaction of the BF acid and ammonia base with cooling and an excess of NH₄F(HF)_x salt to prevent localized over heating. FIG. 1 presents the HF pressure as a function of temperature and selected values of NH₄F(HF)_x solution acidity x. The NH₄F(HF)_x salt solution for transportation preferably has a NH₄F(HF)_x solution acidity x value less than 10, more preferably less than 7, and most preferably less than 5 in order to decrease the shipping container pressure and KU release rate in the event of a containment failure. The shipping container may be advantageously pressurized with a moderate amount of an inert gas, such as dry nitrogen, to prevent ingression of atmospheric contaminates.
• FIG. 2 is a block flow diagram for the fluorine and nitrogen trifluoride production facilities. The system shown in FIG. 2 includes a NF₃ production unit 1, an BF production unit 2, an F₂ production unit 3, an F₂ purification and storage unit 4, an NF₃ purification and storage unit 5, and a facility abatement unit 6, interconnected and operable as will be more fully described below.
• A NH₄F(HF)_x feed stream 7 provides the NH₄F(HF)_x solution to the BF production unit 2, where some of the NH₄F(HF)_x solution is provided to NF₃ production unit 1, via feed line 10. In addition, some of the NH₄F(HF)_x solution is heated, and optionally reacted with F₂ in the HF production unit 2 to produce HF and then provide such HF to F₂ production unit 3, via feed line 11. The optional F₂ for use in the HF production unit 2, is fed via feed line 8, from the F₂ production unit 3. Waste gas from the HF production unit 2, is sent to the facility abatement unit 6, via waste line 13, for proper disposal.
• The F₂ production unit 3, produces a crude F₂ product stream that is sent to the F₂ purification and storage unit 4, via product line 14. In addition, a waste gas, primarily comprising N₂ and H₂, is sent to facility abatement unit 6, for proper disposal via waste line 15. As noted above, some of the F₂ from the F₂ production unit 3, may optionally be sent to HE production unit 2, is fed via feed line 8.
• The F₂ purification and storage unit 4, provides a purified stream of F₂ to the semiconductor plant via feed line 19, and also provides a F₂ feed to NF₃ production unit 1, via feed line 16. Waste gas from the F₂ purification and storage unit 4, is sent to the facility abatement unit 6, via waste line 17, for proper disposal.
• The F₂ provided to NF₃ production unit 1, reacts with a large excess of NH₄F(HF)_x solution provided to the NF₃ production unit 1, via feed line 10, from ET production unit 2. NF₃ is produced in the NF₃ production unit 1 in accordance with the following reaction:
• 
  3F₂+(1+α)NH₄F(HF)_x→NF₃+αNH₄F(HF)_x+(4+x)/α,
• In his formula, α represents the ratio of the NH₄F(HF)_x product rate to its stoichiometric feed rate. Ammonia may be added to the NF₃ production unit 1, to control the NH₄F(HF)_x, melt acidity value x in accordance with the following formula:
• 
  [(4+x)/(x+1)]NH₃+αNH₄(HF)_x+(4+x)/α→[α+(4+x/(x+1)]NH₄F(HF)_x.
• Preferably the NF₃ production unit 1, operates with an NH₄F(HF)_x melt acidity x value between 1.4 and 2.0. In this light, the NH₄F(HF)_x feed stream 7, preferably has a melt acidity x value between 5 and 10. In addition, the NH₄F(HF)_x solution feed provided through feed line 10 preferably has a melt acidity x value between 0 and 1.5, more preferably between 0.25 and 1.25, and most preferably between 0.5 and 1. The waste sent through waste line Stream 12, preferably has a melt acidity x value less than 1, more preferably less than 0.5.
• During the NF₃ production process, corrosion products, such as NiF₂ and CuF₂ from a Monel reactor wall, accumulate in the NH₄F(HF)_x solution and significantly decrease the F₂-to-NF₃ conversion efficiency. Therefore, an NH₄F(HF)_x by-product is removed from the NF₃ production unit 1, to maintain a constant NH₄F(HF)_x melt volume in the NF₃ production unit 1, and to remove the corrosion products. The NH₄F(HF)_x byproduct stream is sent via byproduct line 9, to the HF production unit 2 to produce an appropriate NH₄F(HF)_x feedstock for the NF₃ production unit 1, and to concentrate the non-volatile corrosion products in the a heavy metals discard stream, that is discarded via heavy metal waste line 12.
• The NF₃ production unit 1, also produces a crude NF₃ product that is sent to NF₃ purification and storage unit 5, via product line 18. The NF₃ purification and storage unit 5, provides a purified NF₃ stream to the semiconductor plant via product line 20. Waste gas from the NF₃ purification and storage unit 5, is sent to the facility abatement unit 6, via waste line 21, for proper disposal.
• The facility abatement unit 6, treats the various waste products in an appropriate manner and disposes of the waste via waste line 22.
• As is apparent from the above description, the NH₄F(HF)_x solution used in the NF₃ production unit 1, may be provided as a new feed stream via feed stream 7 and feed line 10, or may be recycled from the NF₃ production unit 1, via byproduct line 9, and feed line 10. As shown in FIG. 2, a single BF production unit 2 (reactor) is utilized in an alternate manner to treat either the NH₄F(HF)_x from feed stream 7 or the byproduct line 9. However, separate reactors could be utilized, particularly since the treatment of the NH₄F(HF)_x solution from feed stream 7 would typically be carried out at different operating conditions than the treatment of NH₄F(HF)_x solution from byproduct line 9. As noted above with respect to NH₄F(HF)_x solution from byproduct line 9, as small portion would be discarded via waste line 12, to remove non-volatile impurities.
• FIG. 3 provides more detail for the HF production unit 2. Where appropriate, like numerals have been used to describe like components as those described with respect to FIG. 2. In particular, the BF production unit 2, includes a reactor 29, having insulation 30, and heater 31. The reactor 29, contains an NH₄F(HF)_x bath 27, and NH₃—HF vapor space 28. As noted above, the NH₄F(HF)_x feed 24, may be either from feed stream 7, or byproduct line 9. In either case the NH₄(HF)_x feed 24, is preferably heated in a heat exchanger 25, and then introduced to the NH₄F(HF)_x bath 27. NH₃—HF vapor is produced by heating the NH₄F(HF)_x bath 27, and occupies NH₃—HF vapor space 28, preferably at a pressure greater than 1 atmosphere and less than 2 atmospheres, more preferably greater than 1.05 atmospheres, and less than 1.5 atmospheres, and most preferably greater than 1.1 atmospheres and less than 1.25 atmospheres. Heater 31, is advantageously used to heat the NH₄F(HF)_x bath 27, and may be placed on reactor 29 wall, as shown in FIG. 3, or alternatively may be submerged in the NH₄F(HF)_x bath 27. The heater 31, may be pipes using a gaseous or liquid heating medium or electrical resistance elements. The operating temperature for the NH₄F(HF)_x bath 27, is preferably between 200 and 240° C. with the NH₃—HF vapor space 28, operating at about 1 atmosphere.
• Since the HF latent heat of vaporization ranges from about 10 to 100 kilo-Joules per gram mole F as the NH₄F(HF)_x melt acidity x value decreases from 20 to about 0.5, the ratio of the flow rate of the HF feed through feed line 11, to the flow rate of NH₄F(HF)_x solution through feed line 33, which is the equivalent to the melt acidity value x, can be most easily controlled by controlling the energy input to the NH₄F(HF)_x feed 24, rate. The higher the energy input, the greater the ratio. The practical limit for the melt acidity value x of the NH₄F(HF)_x solution through feed line 33, and therefore for the ratio is about 0.25. This ratio can be extended beyond this limit by the addition of fluorine from feed line 23, to the HF—NH₃ vapor space 28, where the fluorine reacts with the ammonia vapor to produce primarily nitrogen and hydrogen fluoride. A heat exchanger 32, transfers the large heat of reaction to the NH₄F(HF)_x bath 27, to further facilitate the production of HF vapor with smaller quantities of NH₃ vapor.
• The tempered reactor product 34, comprising NH₃, HF, and NH₄F(HF)_x is fed to a HF purification column 35, such as a rectifying distillation column equipped with packing 37, and condenser 36. The HF purification column 35, produces an appropriate HF feed for feed line 11, and a NH₄F(HF)_x recycle stream 39, that is advantageously added to the NH₄F(HF)_x bath 27. Advantageously, the tempered reactor product 34, may be used as the heat exchange medium for the NH₄F(HF)_x feed, in heat exchanger 25. The NH₄F(HF)_x product 33 on FIG. 3 may be either directed to the NF₃ production unit 1 via stream 10 on FIG. 2 or discarded via stream 12 on FIG. 2 to control the non-volatile impurity level. The fluorine from feed line 23, shown in FIG. 3, may advantageously be provided from the F₂ production unit 3, via feed line 8, as shown in FIG. 2. The feed line 26, allows for NH₄F(HF)_x solution to pass from the heat exchanger 25 to the reactor 29, and feed line 38, allows for product gases to pass from heat exchanger 25 to HF purification column 35.
• One advantage of the present invention is that only the NH₄F(HF)_x feed is required for the F₂ and NF₃ production, whereas the prior art F₂ production technology required a volatile anhydrous HF feed and NF₃ plants required volatile and toxic F₂ and NH₃ feeds. Further, the environmental risk of the NH₄F(HF)_x feed can be adjusted by adjusting the NH₄F(HF)_x melt acidity x value, wherein decreasing the NH₄F(HF)_x melt acidity x value decreases the feedstock safety risk, but also increases the plant operating costs. Therefore, the NH₄F(HF)_x melt acidity x value can be optimized in accordance with plant tolerances and risk profiles.
• The present invention is also advantageous, because the F₂ feed rate to the NF₃ production unit and NF₃ production rate can be changed rapidly and the NF₃ product can be safely stored. Therefore, the F₂ production unit can be sized to operate at an optimum production rate based on the average semiconductor plant cleaning gas requirement. If the instantaneous quantity of fluorine required by the semiconductor plant is less than the average, then the F₂ flow rate to the semiconductor plant would decrease to meet the cleaning gas demand and the balance of the fluorine production would be used for NF₃ production. If the instantaneous quantity of fluorine required by the semiconductor plant was greater than the average, then the F₂ feed to the NF₃ production unit would decrease or stop and the excess cleaning gas demand would be met by NF₃ from storage. Alternatively, the F₂ production unit capacity can be higher than the average cleaning gas demand to either increase the fraction of the total plant cleaning gas requirement being met by lower cost F₂ cleaning gas or to produce NF₃ for other purposes or for sale to other customers, or both.
• It is anticipated that other embodiments and variations of the present invention will become readily apparent to the skilled artisan in the light of the foregoing description, and it is intended that such embodiments and variations likewise be included within the scope of the invention as set out in the appended claims.

Claims (18)

1. An apparatus for the production of NF₃ and F₂ comprising:

an HF production unit having a source for an NH₄F(HF)_x solution connected thereto;

an F₂ production unit connected to said HF production unit;

an F₂ purification unit connected to said F₂ production unit;

an NF₃ production unit connected to said F₂ purification unit and to said HF production unit; and

an NF₃ purification unit connected to said NF₃ production unit.

2. An apparatus according to claim 18, further comprising:

a facility abatement unit connected to each of said HF production, said F₂ production unit; said F₂ purification unit; said NF₃ production unit; and said NF₃ purification unit.

3. An apparatus according to claim 18, wherein said HF production unit is a reactor constructed to contain a bath of said NH₄F(HF)_x solution and a NH₃—HF vapor space;

said reactor including a heater for heating said bath of said NH₄F(HF)_x solution to produce NH₃—HF vapor which occupies said NH₃—HF vapor space.

4. An apparatus according to claim 20, wherein said heater is located on the wall of said reactor.

5. An apparatus according to claim 20, wherein said heater is submerged in said bath of said NH₄F(HF)_x solution.

6. An apparatus according to claim 20, wherein said heater is a series of pipes using a gaseous or liquid heating medium.

7. An apparatus according to claim 20, wherein said heater is an electrical resistance element heater.

8. An apparatus according to claim 20, wherein said operating temperature for said bath of said NH₄F(HF)_x solution is from 200° C. to 240° C.

9. An apparatus according to claim 20, wherein said NH₃—HF vapor is at a pressure from 1 atmosphere to 2 atmospheres.

10. An apparatus according to claim 20, wherein said NH₃—HF vapor is at a pressure from 1.05 atmospheres to 1.5 atmospheres.

11. An apparatus according to claim 20, wherein said NH₃—HF vapor is at a pressure from 1.1 atmospheres to 1.25 atmospheres.

12. An apparatus according to claim 20, wherein said bath of said NH₄F(HF)_x solution is provided to said reactor from said source of NH₄F(HF)_x solution.

13. An apparatus according to claim 20, wherein said bath of said NH₄F(HF)_x solution is provided from said NF₃ production unit.

14. An apparatus according to claim 20, wherein said bath of NH₄(HF)_x solution is preheated using a heat exchanger prior to being introduced to said reactor and said heating medium used in said heat exchanger is said NH₃—HF vapor.

15. An apparatus according to claim 20, further including a source for F₂ connected to said reactor.

16. An apparatus according to claim 32, further including a heat exchanger in said reactor for transferring the heat of reaction between NF₃ vapor and F₂ provided to said reactor, to the bath of said NH₄F(HF)_x solution.

17. An apparatus according to claim 20, further including an HF purification column connected to a product outlet of said reactor, said HF purification column.

18. An apparatus according to claim 34, wherein said HF purification column is a rectifying distillation column having packing and a condenser.

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