Abstract:
The present invention relates to a process for production, shipment, and treatment of a NH 4 F(HF) x  feedstock for local production of fluorine and NF 3  for semiconductor chamber cleaning without the need for storage of large quantities of dangerous feeds and intermediate products.

Description:
[0001]    This application claims priority from U.S. Provisional application Ser. No. 60/561,180 filed Apr. 9, 2004. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method for local production of fluorine (F 2 ) and nitrogen trifluoride (NF 3 ) 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 NH 4 F(HF) x  solution that can be safely shipped to a F 2  and NF 3  production facility and for the conversion of the NH 4 F(HF) x  solution to the F 2  and NF 3  products. 
       BACKGROUND OF THE INVENTION 
       [0003]    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 NF 3 , SF 6 , C 2 F 6 , or CF 4 , which is usually diluted with argon or helium. 
         [0004]    Unfortunately, SF 6 , NF 3 , C 2 F 6 , and CF 4  have very high global warming potentials, i.e. respectively about 23,900, 10,090, 9,200, 6,500 times CO 2  on a weight average basis over a 100 year time-frame, respectively. While some fluorine containing cleaning gases have much lower global warming potentials, F 2  and ClF 3  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. 
         [0005]    NF 3  is the most common chamber cleaning gas and is typically produced by the reaction of fluorine with a NH 4 F(HF) x  salt, such as by the following reaction: 
         [0000]      3F 2 +NH 4 F(HF) x →NF 3 +(4 +x )HF. 
         [0000]    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, NF 3  production from urea and fluorine has been proposed (as shown in U.S. Pat. No. 6,821,496) using the following key step: 
         [0000]      2CO(NH 2 ) 2 +3F 2 →NF 3 +NH 2 CONHCONH 2 +3HF. 
         [0006]    All these ammonia-based NF 3  production processes use half of the fluorine feed to produce NF 3  and the other half to produce HF. Therefore, the direct use of fluorine as a chamber cleaning gas would be much more efficient than NF 3 . 
         [0007]    Although F 2  is a more efficient and theoretically lower cost chamber cleaning gas than NF 3 , 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. 
         [0008]    First, the quantity of the fluorine product that can be safely stored is severely limited by fluorine&#39;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 NF 3 . 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 NF 3  supply system. In such a case, the theoretical cost savings can not be realized. 
         [0009]    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 
       [0010]    The present invention overcomes the disadvantages noted above by providing a method for remote preparation of a NH 4 F(HF) x  solution that may be safely shipped to a F 2 —NF 3  production facility and for converting the shipped NH 4 F(HF) x  solution to a NH 4 F(HF) x  feed appropriate for NF 3  production and to a HF feed appropriate for F 2  production. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is a plot of the logarithm of the hydrogen fluoride vapor pressure as, a function of temperature with parameters of NH 4 F(HF) x  melt acidity x value. 
           [0012]      FIG. 2  is a block diagram of a F 2  and NF 3  production facility. 
           [0013]      FIG. 3  is a simplified process flow diagram for a method according to the present invention to convert the NH 4 F(HF) x  solution for shipment to appropriate NF 3  plant NH 4 F(HF) x  feed and F 2  plant HP feed. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    In accordance with the present invention an NH 4 (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 NH 4 F(HF) x  salt solution is produced by the reaction of the BF acid and ammonia base with cooling and an excess of NH 4 F(HF) x  salt to prevent localized over heating.  FIG. 1  presents the HF pressure as a function of temperature and selected values of NH 4 F(HF) x  solution acidity x. The NH 4 F(HF) x  salt solution for transportation preferably has a NH 4 F(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. 
         [0015]      FIG. 2  is a block flow diagram for the fluorine and nitrogen trifluoride production facilities. The system shown in  FIG. 2  includes a NF 3  production unit  1 , an BF production unit  2 , an F 2  production unit  3 , an F 2  purification and storage unit  4 , an NF 3  purification and storage unit  5 , and a facility abatement unit  6 , interconnected and operable as will be more fully described below. 
         [0016]    A NH 4 F(HF) x  feed stream  7  provides the NH 4 F(HF) x  solution to the BF production unit  2 , where some of the NH 4 F(HF) x  solution is provided to NF 3  production unit  1 , via feed line  10 . In addition, some of the NH 4 F(HF) x  solution is heated, and optionally reacted with F 2  in the HF production unit  2  to produce HF and then provide such HF to F 2  production unit  3 , via feed line  11 . The optional F 2  for use in the HF production unit  2 , is fed via feed line  8 , from the F 2  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. 
         [0017]    The F 2  production unit  3 , produces a crude F 2  product stream that is sent to the F 2  purification and storage unit  4 , via product line  14 . In addition, a waste gas, primarily comprising N 2  and H 2 , is sent to facility abatement unit  6 , for proper disposal via waste line  15 . As noted above, some of the F 2  from the F 2  production unit  3 , may optionally be sent to HE production unit  2 , is fed via feed line  8 . 
         [0018]    The F 2  purification and storage unit  4 , provides a purified stream of F 2  to the semiconductor plant via feed line  19 , and also provides a F 2  feed to NF 3  production unit  1 , via feed line  16 . Waste gas from the F 2  purification and storage unit  4 , is sent to the facility abatement unit  6 , via waste line  17 , for proper disposal. 
         [0019]    The F 2  provided to NF 3  production unit  1 , reacts with a large excess of NH 4 F(HF) x  solution provided to the NF 3  production unit  1 , via feed line  10 , from ET production unit  2 . NF 3  is produced in the NF 3  production unit  1  in accordance with the following reaction: 
         [0000]      3F 2 +(1+α)NH 4 F(HF) x →NF 3 +αNH 4 F(HF) x+(4+x)/α , 
         [0000]    In his formula, α represents the ratio of the NH 4 F(HF) x  product rate to its stoichiometric feed rate. Ammonia may be added to the NF 3  production unit  1 , to control the NH 4 F(HF) x , melt acidity value x in accordance with the following formula: 
         [0000]      [(4 +x )/( x+ 1)]NH 3 +αNH 4 (HF) x+(4+x)/α →[α+(4 +x /( x+ 1)]NH 4 F(HF) x . 
         [0020]    Preferably the NF 3  production unit  1 , operates with an NH 4 F(HF) x  melt acidity x value between 1.4 and 2.0. In this light, the NH 4 F(HF) x  feed stream  7 , preferably has a melt acidity x value between 5 and 10. In addition, the NH 4 F(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. 
         [0021]    During the NF 3  production process, corrosion products, such as NiF 2  and CuF 2  from a Monel reactor wall, accumulate in the NH 4 F(HF) x  solution and significantly decrease the F 2 -to-NF 3  conversion efficiency. Therefore, an NH 4 F(HF) x  by-product is removed from the NF 3  production unit  1 , to maintain a constant NH 4 F(HF) x  melt volume in the NF 3  production unit  1 , and to remove the corrosion products. The NH 4 F(HF) x  byproduct stream is sent via byproduct line  9 , to the HF production unit  2  to produce an appropriate NH 4 F(HF) x  feedstock for the NF 3  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 . 
         [0022]    The NF 3  production unit  1 , also produces a crude NF 3  product that is sent to NF 3  purification and storage unit  5 , via product line  18 . The NF 3  purification and storage unit  5 , provides a purified NF 3  stream to the semiconductor plant via product line  20 . Waste gas from the NF 3  purification and storage unit  5 , is sent to the facility abatement unit  6 , via waste line  21 , for proper disposal. 
         [0023]    The facility abatement unit  6 , treats the various waste products in an appropriate manner and disposes of the waste via waste line  22 . 
         [0024]    As is apparent from the above description, the NH 4 F(HF) x  solution used in the NF 3  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 NF 3  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 NH 4 F(HF) x  from feed stream  7  or the byproduct line  9 . However, separate reactors could be utilized, particularly since the treatment of the NH 4 F(HF) x  solution from feed stream  7  would typically be carried out at different operating conditions than the treatment of NH 4 F(HF) x  solution from byproduct line  9 . As noted above with respect to NH 4 F(HF) x  solution from byproduct line  9 , as small portion would be discarded via waste line  12 , to remove non-volatile impurities. 
         [0025]      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 NH 4 F(HF) x  bath  27 , and NH 3 —HF vapor space  28 . As noted above, the NH 4 F(HF) x  feed  24 , may be either from feed stream  7 , or byproduct line  9 . In either case the NH 4 (HF) x  feed  24 , is preferably heated in a heat exchanger  25 , and then introduced to the NH 4 F(HF) x  bath  27 . NH 3 —HF vapor is produced by heating the NH 4 F(HF) x  bath  27 , and occupies NH 3 —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 NH 4 F(HF) x  bath  27 , and may be placed on reactor  29  wall, as shown in  FIG. 3 , or alternatively may be submerged in the NH 4 F(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 NH 4 F(HF) x  bath  27 , is preferably between 200 and 240° C. with the NH 3 —HF vapor space  28 , operating at about 1 atmosphere. 
         [0026]    Since the HF latent heat of vaporization ranges from about 10 to 100 kilo-Joules per gram mole F as the NH 4 F(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 NH 4 F(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 NH 4 F(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 NH 4 F(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—NH 3  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 NH 4 F(HF) x  bath  27 , to further facilitate the production of HF vapor with smaller quantities of NH 3  vapor. 
         [0027]    The tempered reactor product  34 , comprising NH 3 , HF, and NH 4 F(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 NH 4 F(HF) x  recycle stream  39 , that is advantageously added to the NH 4 F(HF) x  bath  27 . Advantageously, the tempered reactor product  34 , may be used as the heat exchange medium for the NH 4 F(HF) x  feed, in heat exchanger  25 . The NH 4 F(HF) x  product  33  on  FIG. 3  may be either directed to the NF 3  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 F 2  production unit  3 , via feed line  8 , as shown in  FIG. 2 . The feed line  26 , allows for NH 4 F(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 . 
         [0028]    One advantage of the present invention is that only the NH 4 F(HF) x  feed is required for the F 2  and NF 3  production, whereas the prior art F 2  production technology required a volatile anhydrous HF feed and NF 3  plants required volatile and toxic F 2  and NH 3  feeds. Further, the environmental risk of the NH 4 F(HF) x  feed can be adjusted by adjusting the NH 4 F(HF) x  melt acidity x value, wherein decreasing the NH 4 F(HF) x  melt acidity x value decreases the feedstock safety risk, but also increases the plant operating costs. Therefore, the NH 4 F(HF) x  melt acidity x value can be optimized in accordance with plant tolerances and risk profiles. 
         [0029]    The present invention is also advantageous, because the F 2  feed rate to the NF 3  production unit and NF 3  production rate can be changed rapidly and the NF 3  product can be safely stored. Therefore, the F 2  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 F 2  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 NF 3  production. If the instantaneous quantity of fluorine required by the semiconductor plant was greater than the average, then the F 2  feed to the NF 3  production unit would decrease or stop and the excess cleaning gas demand would be met by NF 3  from storage. Alternatively, the F 2  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 F 2  cleaning gas or to produce NF 3  for other purposes or for sale to other customers, or both. 
         [0030]    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.