Abstract:
A method for selecting, using, regenerating, and recycling fluorochemicals for plasma cleaning and etching steps for semiconductor device production is provided. This method comprises selection of a regenerable fluorochemical, using the regenerable fluorochemical working fluid for chamber cleaning or etching, and regenerating the fluorochemical working fluid with sequential fluorination and defluorination steps. This invention allows the efficient use of the most cost-effective and safe chamber cleaning gases without adversely affecting the environment.

Description:
[0001]     This application claims priority from U.S. Provisional application Ser. No. 60/519,203 filed Nov. 12, 2003 and 60/556,912 filed Mar. 26, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a method to increase cleaning and etching gas utilization in the production of semiconductor electronic devices and thin film transistor liquid crystal display devices. More specifically this invention relates to a method for selecting materials for chamber cleaning or etching, and treating the gases evolved from semiconductor production processes to regenerate the fluorochemical working fluid.  
       BACKGROUND OF THE INVENTION  
       [0003]     Production processes for semiconductor electronic devices and thin film transistor liquid crystal display devices generally use gaseous fluorochemicals, such as nitrogen trifluoride (NF 3 ), sulfur hexafluoride (SF 6 ) and carbon tetrafluoride (CF 4 ), as etchants and chamber cleaning gases. In etching production steps, silicon is removed from the semiconductor device by reaction with fluorine from the gaseous fluorochemicals to produce primarily SiF 4 . In chamber cleaning steps, the deposits on the wall of the process chamber are removed by reaction with fluorine from the gaseous fluorochemical to produce predominately SiF 4 , HF, and CF 4  from the silicon, hydrogen, and carbon atoms in the debris on the process chamber wall. The consumption of these gases within the process chamber is typically low, usually less than twenty percent. Typically, combustion with a hydrocarbon fuel has been used to convert the fluorine atoms within the fluorochemical species that are present in the effluent stream from the process chamber cleaning and etching process steps to HF. There are two primary problems with this approach. First, the HF product from a conventional hydrocarbon combustion processes problem. Second, the available combustion exhaust abatement systems typically achieve incomplete conversion and recovery of the most stable fluorochemical gases, particularly SF 6  and CF 4 . As a result, a substantial portion of the original fluorochemical feed gas can ultimately be discharged to the atmosphere. These very stable fluorochemical etching and chamber cleaning gases have long persistence in the atmosphere, very effectively absorb infrared radiation, and, as a result, have a disproportionately large contribution to the global warming problem. As a result, the semiconductor industry has sought ways to diminish the impact of chamber cleaning and etching gas emissions.  
         [0004]     Three general strategies have emerged to decrease global warming gas emissions in the semiconductor industry. First, innovative approaches have been proposed to use fluorochemicals that have much lower persistence in the atmosphere and, therefore, much lower global warming potential. For example, U.S. Application 2003/0010354 teaches a chamber cleaning method that utilizes molecular fluorine to avoid the use of fluorochemical gases having higher global warming potential like NF 3 , CF 4 , C 2 F 6 , and SF 6 . One disadvantage of this approach is that fluorochemical gases with the lowest global warming potential, F 2  and ClF 3  for example, are generally very reactive and toxic, thus adversely effecting plant safety.  
         [0005]     Second, the industry has sought to develop more effective exhaust abatement technology. For example, U.S. Pat. No. 5,965,786 teaches a method of using atmospheric pressure plasma to destroy unused perfluorinated and/or hydrofluorocarbon chamber cleaning and etching gases. A significant difficulty with this approach is that these plasma processes have also exhibited incomplete conversions and poor selectivity.  
         [0006]     Third, innovators have tried various approaches to recover, purify, and recycle the cleaning gases. For example, U.S. Pat. No. 6,032,484 teaches a method for separation and recovery of fluorochemicals using membranes. A disadvantage of this approach is that the chamber effluent is highly diluted and a complex mixture of unconverted cleaning gases and very reactive by-products like molecular fluorine make abatement and recycle in this manner very difficult and inefficient.  
         [0007]     U. S. Patent Application 2003/0056388 teaches that very high etch rates, efficient cleaning, and excellent process cost performance can be achieve with F 2 —SF 6 , NF 3 —SF 6 , and O 2 —SF 6  mixtures. Unfortunately, only a fraction of the SF 6  feed is consumed in the process chamber and SF 6  has a very high global warming potential and is difficult to either destroy or recover and recycle.  
         [0008]     Therefore, there remains a need in the art related to the efficient use of fluorochemicals in semiconductor processing techniques. There also remains a need to find cost-effective solutions for the problems noted above with respect to the prior art.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention teaches a method for selecting, using, regenerating, and recycling fluorochemicals for plasma cleaning or etching steps in the production of semiconductor devices. More specifically, the method of the present invention comprises the selection of a regenerable fluorochemical working fluid and more active fluorochemicals, using the regenerable fluorochemical working fluid for chamber cleaning or etching, regenerating the fluorochemical working fluid with the more active fluorochemical in a two stage process comprising a sequential fluorination step and a subsequent defluorination step, and recovering and recycling the regenerated fluorochemical working fluid. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a block flow diagram for the method of the present invention.  
         [0011]      FIG. 2  is a simplified process flow diagram for a first example of the method according to the present invention.  
         [0012]      FIG. 3  is a block flow diagram for a second example of the method according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     The ideal chamber cleaning gas has a low reactivity at ambient temperature, is reasonably stable at elevated temperature, has low toxicity, and has low global warming potential. Table 1 summaries useful metrics to characterize fluorochemical etching and chamber cleaning gases and define recovery and recycle processes. The following discussion will consider each of these parameters.  
                                                                                         TABLE 1                           Fluorochemical Cleaning and Etching Gas Characterization Parameters                TLV-               ACGIH 2     Temperature, ° C. (see note 3 for K 1  definition)            Specie   GWP 1     ppm   T 1  (K 1  = 10 −22 )   T 2  (K 1  = 10 −13 )   T 3  (K 1  = 10 −4 )   T 4  (ΔG = 0)                    NF 3     10,090   10   171   363   844   672       CF 4     6,500   asphyxiant   651   1034   1962   &gt;&gt;2000       ClF 3     low   very toxic   45   113   231   74       CH 2 F 2     150   1000   590   952   1833   always &gt; 0       SF 6     23,900   1000   384   648   1270   &gt;&gt;2000       F 2     low   1   21   160   535   always = 0                   1 100 year global potential relative to CO 2  (weight basis)              2 American Conference of Governmental Industrial Hygienists (ACGIH) ambient threshold limit values (TLVs)              3 K 1  is the first fluorine atom dissociation equilibrium constant [NF 3             NF 2  (g) + F (g)]            
 
         [0014]     Many fluorochemicals have unusually long lifetimes in the atmospheres and unusually high infrared absorptivity, which leads to a very large impact on global warming. The global warming potential (GWP) is used to assess the relative impact of alternative gases on global warming due to infrared absorption over a 100 year timeframe relative to CO 2  using a weight comparison basis. For example, as set forth in Table 1, one gram of NF 3  would have roughly the same impact as 10,090 grams of CO 2 . The fluorochemicals listed in Table 1 have a very broad range of global warming potential from less than CO 2  for F 2  and ClF 3  to 23,900 times the CO 2  global warming potential for SF 6 .  
         [0015]     The American Conference of Governmental Industrial Hygienists (ACGIH) has established ambient concentrations (ppm) threshold limit values (TLVs). Most workers can be repeatedly exposed to chemicals at the TLV day after day without adverse health effects. Therefore, the ACGIH-TLV value provides a useful measure of relative toxicity. An asphyxiant causes a health hazard by decreasing the oxygen content from about 21 to less than 19.5 volume percent, which would be equivalent to about 15,000 ppmv. As shown in Table 1, the ACGIH-TLV values for the listed fluorochemicals range from about 1 ppm for F 2  to about 15,000 ppmv (asphyxiant) for CF 4 .  
         [0016]     There is no widely accepted measure of fluorochemical fluorination reactivity. However, there is general agreement that the fluorine radical, F(g), is primarily responsible for the extraordinary reactivity of fluorochemicals under certain circumstances. Therefore, the first fluorine dissociation constant seems to provide a useful measure of the fluorochemical reactivity. The Outokumpu HSC Chemistry for Windows computer program (Version 4) provides a convenient method to estimate the first fluorine dissociation constant for a wide range of compounds. For example, for NF 3 , the first dissociation reaction is NF 3 (g)→NF 2 (g)+F(g) and the first fluorine dissociation constant may be calculated from  
           K     1   ,     NF   3         =         P     NF   2       ⁢     P   F         P     NF   3           ,       
 
 where P NF     2   , P F , &amp; P NF     3    are the partial pressures in atmospheres of NF 2 (g), F(g), and NF 3 (g), respectively. It has been found that temperatures at which the first fluorine dissociation constants (K 1 ) are equal to 10 −22 , 10 −13 , and 10 −4  atmospheres (T 1 , T 2 , and T 3  respectively) provide useful measures of the fluorocarbon reactivity and guidance for selecting reactor operating conditions for the present invention. The value of K 1  for molecular fluorine at room temperature (about 10 −22 ) provides a useful and familiar benchmark for a modest fluorination rate. T 1  values for other fluorochemicals (some as noted in Table 1) are then calculated based on the K 1  value of 10 −22 . Practical experience suggests that K 1  values of greater than about 10 −13  result in fluorination rates that are appropriate for abatement processes for a wide variety of fluorochemical species. Therefore, T 2  provides an estimate of the temperature required to achieve adequate fluorination rates for a wide range of fluorochemicals and also provides a reasonable estimate for the minimum temperature for thermal abatement process. It should be noted that fluorocarbons with lower T 2  values have higher fluorination activity. The stability of the fluorochemical can then be assessed in two ways. T 3  is the temperature required for dissociating about one percent of the fluorochemical, assuming there are no disproportionation reactions. T 4  is the temperature at which the Gibes free energy of formation for the specie, from the most stable reactants, become positive, which is a measure of the disproportionation tendency. For example, the most stable reactants for CH 2 F 2  would be HF and carbon and the most stable reactants for NF 3  would be N 2  and F 2 . T 3  and T 4  are used as measures of the maximum practical working fluorochemical temperature. 
 
         [0017]     The data in Table 1 clearly shows a difficulty associated with selecting fluorochemical process chamber cleaning and etching gases. In particular, only relatively complex and expensive reagents like CH 2 F 2  are stable, and have both reasonable toxicity and global warming potentials. The less costly reagents have varying problems. For example, some simple reagents are both highly toxic and very reactive, e.g. ClF 3  or F 2 . Other fluorochemicals are stable but have high global warming potentials, e.g. NF 3 , SF 6  or CF 4 . The present invention provides a method for the use of a combination of more reactive and less reactive fluorochemicals to cost effectively and simultaneously achieve safety and environmental goals.  
         [0018]      FIG. 1  is a block flow diagram for the method according to the present invention wherein a combination of more reactive and less reactive fluorochemical gases is utilized. In particular, in accordance with the present invention, the less reactive fluorochemical gas is recycled for further use in the semiconductor manufacturing process, thus increasing the use efficiency of such gas, and reducing the impact of global warming potential emissions. Further, by recycling the less reactive fluorochemical gas, significant cost savings can be realized.  
         [0019]     For purposes of the present invention, a fluorochemical is a chemical compound that contains some fluorine atoms. Less reactive fluorochemical gas means a gas having a higher T 2  value as compared with the more reactive fluorochemical gas. Preferably the T 2  value for the less reactive fluorochemical is at least 10° C. greater than the T 2  value for the more reactive fluorochemical gas, more preferably at least 20° C. higher, and most preferably at least 50° C. higher.  
         [0020]     As Shown in  FIG. 1 , the recycled less reactive fluorochemical gas  1  is fed to a semiconductor process chamber  3 . Less reactive fluorochemical gas or gases  2  may be added or removed to adjust for consumption or excess production of less reactive fluorochemical gas or gases  1 . The less reactive fluorochemical gas may be a substantially pure fluorochemical, a mixture of fluorochemicals, or a mixture of fluorochemicals and non-fluorochemicals, usually inert gases like argon, nitrogen, or helium. The less reactive fluorochemical gas  1  or  2  must contain a non-fluorine atom and have a T 4  value that is greater than T 2 . For example and referring to Table 1, ClF 3  can not be considered a less reactive fluorochemical gas because T 2  is greater than T 4 . F 2  can not be considered a less reactive fluorochemical because it does not contain any non-fluorine bonds. NF 3  could be considered a less reactive fluorochemical relative to F 2  or ClF 3 , but not relative to CF 4  or SF 6  because its T 2  value is less than the values of T 2  for CF 4  or SF 6 . The less reactive fluorochemical  1  may be vaporize by subjecting the less reactive fluorochemical  1  to an elevated temperature or reduced pressure. A supplemental process chamber feed gas  4  can be added to the less reactive fluorochemical gas  1  prior to entering the process chamber  3  or fed directly to the process chamber  3 . The supplemental feed gas  4  typically contains inert gases, e.g. argon, or helium, to help sustain the plasma and could contain other non-fluorochemical cleaning or etching gases, e.g. N 2 O, O 2 , Cl 2 , or a more reactive fluorochemical gas, i.e. a gas with a T 2  value less than the major fluorochemical components of the less reactive fluorochemical gas  1 . Alternatively, the process chamber supplemental feed gas  4  could contain components found in the less reactive fluorochemical gas  1  and serve to make-up for relatively small quantities that are lost or consumed in the semiconductor manufacturing process. A conventional remote plasma chamber may be used to subject the less reactive fluorochemical gas  1  and supplemental feed gas  4  to a plasma discharge either in the process chamber  3  or prior to entering the process chamber  3 . The remote or process chamber plasma may also be used to produce the more reactive fluorochemical species from the less reactive gas and supplemental gas feed. For example, remote or process chamber plasmas could convert a feed comprising SF 6  and O 2  to more reactive fluorine radicals and molecular fluorine fluorochemical species in addition to the less reactive SF 6  fluorochemical. The process chamber  3  typically operates in various types of process steps; such as selective deposition of thin film layers and etching thin films to produce thin film transistor semiconductor devices. Periodically, the process chamber  3  must be cleaned to prevent contamination of the thin film layers with debris that inevitably accumulates on the process chamber walls. In accordance with the present invention, a combination of less reactive fluorochemical gas and more reactive fluorochemical gas is used for the cleaning operation. Alternatively, a mixture of less reactive fluorochemical gas and more reactive fluorochemical gas can be used for etching operations carried out in the process chamber  3 . A conventional vacuum pump is used to increase the pressure of the process chamber  3  product gases to roughly atmospheric pressure. The method of the present invention further provides for regeneration of the less reactive fluorochemical gas from the off-gas  5  from the process chamber  3 .  
         [0021]     The process chamber off-gas  5  is typically a complex mixture of unreacted etching or cleaning gases, e.g. CF 4 , SF 6 , O 2 , F 2 , or NF 3 ; etching or cleaning by-products, e.g. CF 4 , CO 2 , SiF 4 , or AlF 3 ; and fluorochemical decomposition products, e.g. F 2 , OF 2 , or SO x F y  where y is 1, 2, 3, or 4 and the sum of two times x and y is less than or equal to six. Many of the fluorochemical components in the off-gas  5  are highly reactive, toxic or have high global warming potentials. Therefore, it is desirable to covert the more problematic components of the off-gas  5  to other components that pose less risk or that can be recycled and reused in other processes carried out in the process chamber  3 .  
         [0022]     According to the present invention, the off-gas  5  is subjected to a fluorination step  6  using either the more reactive fluorochemical species (T 2  values less than the T 2  values of the less reactive fluorochemical gas  1 ) of the process chamber off-gas  5  or optionally a fluorination step supplemental feed  7 . The supplemental fluorination step feed  7  can be a non-hydrogen containing fuel, e.g. carbon, sulfur, SO 2  or CS 2 , a sub-fluorinated fluorochemical, e.g. SO 2 F 2 , or more reactive fluorochemical, e.g. F 2  or NF 3 . The fluorination step  6  operates at roughly the off-gas  5  pressure, usually about atmospheric pressure. The fluorination step  6  preferred temperature is greater than T 2  for the more reactive fluorochemical components in the process chamber  3  off-gas  5  and less than the lesser of T 3  or T 4  for the major components in the less reactive fluorochemical gas  1 . The fluorination step  6  preferred temperature range may be achieved by indirect conduction heating, direct heating with an arc discharge, indirect inductive coupled heating, atmospheric pressure plasma discharge, exothermic reactions, e.g. ½S 2 (g)+3F 2 (g)→SF 6 (g) or SO 2 +3F 2 →SF 6 +O 2 , or other conventional heating means. However, combustion with a hydrogen containing fuel (hydrocarbon); may not be used to heat the off-gas  5 . The fluorination step  6  is carried out within a closed reactor vessel with a feed and product gas connections. Metal packing may be advantageously added to the fluorination step reactor to increase the fluorination reaction and heat transfer rates. Nickel, Monel, and copper are preferred materials of construction for the fluorination step packing. In some cases, the process chamber  3  off-gases  5  are fully appropriate feeds to the fluorination step  6 . In these cases, no high temperature fluorination step supplemental feed  7  is required. The off-gas  5  and optional supplemental fluorination step feed  7  are contacted in fluorination step  6  until the very rapid fluorination reactions, e.g. F 2 (g)+SF 4 (g)→2 SF 6  (g), are essentially complete to produce the high temperature fluorination product gas  8 . The gas residence time in the fluorination step  6  is preferably between 0.25 and 20 seconds, more preferably between 0.5 and 10 seconds, and most preferably between 1 and 5 seconds. The gas residence time is defined as the ratio of the fluorination step reactor void volume divided by the average total gas volumetric feed rate at the average fluorination step temperature and pressure. The fluorination step pressure is preferably between 0.5 and 3 atmospheres, more preferably between 0.75 and 2 atmospheres, most preferably about one atmosphere. By carrying out the fluorination step  6 , some of the more reactive fluorochemicals gas in the process chamber  3  off-gas  5  are converted to the preferred less reactive fluorochemical gas  1 . Therefore, the product gas  8  contains a greater percentage of the less reactive fluorochemical gas  1 , than the off-gas  5  and in particular more of the fluorochemical gas to be recycled.  
         [0023]     The product gas  8  is then subjected to a defluorination step  10  using a fluorine atom getter  9  to selectively reduce the concentration of the more reactive fluorochemical components, e.g. F 2 , OF 2 , SiF 4 , in the product gas  8 . The operating pressure of the defluorination step  10  is preferable greater than 50%, more preferably greater than 75% and most preferably greater than 90% of the fluorination step. The preferred defluorination step  10  operating temperature is less than T 2  for the major fluorochemical components in the less reactive fluorochemical gas  1  and greater than T 1  for the more reactive fluorochemical gas. The gas residence time is adjusted to essentially completely convert the more reactive fluorochemicals with very little loss of the less reactive fluorochemical components in the less reactive fluorochemical gas  1 . Preferably greater than 80%, more preferably greater than 90%, most preferably greater than 95% of the more reactive fluorochemical is removed with preferably less than 20%, more preferably less than 10%, most preferably less than 5% of the less reactive fluorochemical removed. The gas residence time in the defluorination step  7  is preferably between 0.1 and 10 seconds, more preferably between 0.5 and 5 seconds, and most preferably between 1 and 3 seconds. The fluorine atom getter  9  may be any material or combination of materials that can be easily separated from the low temperature fluorination product  11 , reacts with fluorine radicals, e.g. F(g), or reactive components, e.g. SiF 4 , at modest temperatures, and produces a useful fluorochemical product or a fluorochemical waste product that can be easily removed and conveniently discarded. For example, elemental sulfur or an ammonium hydrogen fluoride melt could be used as a fluorine atom getter  9  and produce SF 6  or NF 3  products, respectively which can be recycled and reused. Alternatively an alumina bed could be used as a fluorine atom getter  9  and produce an AlF 3  waste product, which would remain in the alumina bed for easy recovery as a low temperature fluorination step waste product  12 , and O 2  which would be a component in the low temperature fluorination product  11 . Alternatively, water or an aqueous solution could be used as the stream  9  feed for the defluorination step  10  to remove F 2 , SiF 4 , SO x F y  compounds by hydrolysis. The reactive nature of the fluorine radical provides many potential fluorine atom getter  9  options; including the use of a combination of fluorine atom getters or sequential use of fluorine atom getters. For example, one could sequentially contact the product gas  8  with sulfur, to produce SF 6 , and then with alumina, to virtually eliminate the remaining more reactive fluorochemicals from low temperature fluorination product  11 .  
         [0024]     The defluorination product  11  is then subjected to a recycle fluorochemical recovery step  13  to recover the less reactive fluorochemical gas  1 . Preferably, the defluorination product  11  is compressed to the distribution pressure of the less reactive fluorochemical gas  1 , usually 2 to 5 atmospheres, and a cryogenic distillation is carried out to recover the less reactive fluorochemical gas  1  from the defluorination product  11 . Any high freezing point components, e.g. H 2 O and HF, may be advantageously removed using an appropriate adsorbent, e.g. alumina prior to the distillation step.  
         [0025]     The recovered less reactive fluorochemical gas  1  is recycled to the process chamber  3  for further semiconductor processing use. Distillation by-products  14  may be routed for further abatement, recycle or disposal as appropriate.  
         [0026]     In accordance with the present invention, manufacturers can determine the composition of off-gas  5  based upon the process being carried out in the process chamber  3  and the process gases utilized. With this in mind, key operating conditions, e.g. temperature, for the fluorination step  6  and the defluorination step  10  can be determined using the T 2  values for the fluorochemical components in the off-gas  5 . The following examples provide several embodiments using the method according to the present invention to illustrate how the method of the present invention operates, but are not intended to limit the scope of the present invention.  
       EXAMPLE 1  
       [0027]      FIG. 2  will be used to illustrate an embodiment of the present invention that uses a He—NF 3 —SF 6 —O 2  feed to process chamber  3  that is operating in the etch mode. The supplemental feed gas  4  is comprised of a helium feed  15 , oxygen feed  16 , and NF 3  feed  17 . An SF 6  recycle gas feed  18  is either combined with the supplemental feed gas  4  or fed separately to the process chamber  3 . Typical helium-to-NF 3 , SF 6 -to-NF 3 , O 2 -to-NF 3  process chamber  3  molar feed ratios are 12/1 and 6/1, and 0.5/1, respectively. The process chamber  3  operates at about 10 millibar using a microwave excitation source. The process chamber  3  off-gas  19  is directed to a vacuum pump  20  to maintain the process chamber  3  pressure and increase the off-gas  19  pressure to about atmospheric pressure. A portion of the less reactive fluorochemical gas  21  may be advantageous used as ballast gas for the vacuum pump  20 , replacing the conventional nitrogen. This is advantageous because the use of nitrogen as the ballast gas increases the operating pressure or reflux rate required to prevent freezing in the subsequent cryogenic purification system. The flow rate for the less reactive fluorochemical gas  21  is typically about 30% of the off-gas  19  molar flow rate.  
         [0028]     The vacuum pump off-gas  5  is fed to the fluorination step  6 . Elemental sulfur  22  may optionally be fed to the high temperature fluorination step  6  to produce make-up SF 6 . Alternatively, make-up SF 6  may be added to the supplemental feed gas  4 . The fluorination step  6  is conducted in a vessel equipped with a copper or nickel reaction bed  23 , electrical heater  24  and insulation  25 . The fluorination step  6  uses molecular fluorine to produce SF 6  from less oxidized sulfur species in the off-gas  5 . The process chamber  3  produced the fluorine reactants for the fluorination step  6  primarily from the NF 3  feed  17 . As previously noted, the fluorination step  6  operating temperature is chosen to be greater than T 2  for the more reactive fluorochemical components of the off-gas  5  and less than the lesser of T 3  or T 4  for the less reactive components of off-gas  5 . In this example, F 2  is the more reactive fluorochemical component and SF 6  and NF 3  are the less reactive fluorochemical components. Therefore, the appropriate temperature for the fluorination step  6  is between the T 2  value for F 2 , e.g. about 200° C. and the least of the T 3  or T 4  values for SF 6  or NF 3 , e.g. about 500° C. The gas residence time should be about two seconds at this temperature to ensure essentially complete conversion of the S 2 F 10 , SOF 2 , SOF 4  type species to SF 6 .  
         [0029]     The fluorination step product gas  8  is cooled to a temperature slightly less than the defluorination step  10  operating temperature in the fluorination step intercooler  26  to produce the defluorination step feed  27 . The defluorination step  10  is conducted in a vessel equipped with a gas preheater bed  28 , fluoride getter bed  29 , electrical heater  24  and insulation  25 . The gas preheater bed  28  enhances heat transfer in order to increase the gas temperature uniformity prior to contacting gases with an activated alumina fluoride getter bed  29 . The fluoride getter bed  29  consumes essentially all the fluorine and other reactive components to concentrations less than 1000 ppm without significant conversion of NF 3  or SF 6 . As previously noted, the operating temperature for the defluorination step  10  is less than the T 2  value for the less reactive fluorochemical components of the off-gas  5 . In this example, the operating temperature should be less than the T 2  value for NF 3  or SF 6 , e.g. between 100° C. and 200° C. The gas residence time in the fluoride getter bed  29  should be about 2 seconds. The activated alumina fluoride getter bed  29  material must be periodically replaced, and two fluoride getter beds  29  may be advantageously used, in countercurrent flow pattern with the defluorination step feed  27 , to increase the utilization of fluoride getter without aversely affecting conversion.  
         [0030]     Then the defluorination step product  11  is cooled in the fluorination step aftercooler  30 . The cooled defluorination step product  31  is introduced into the distillation feed compressor  32  to increase the pressure to an appropriate distribution pressure and avoid freezing in the recycle fluorochemical recovery step using cryogenic distillation. The distillation feed gas  33  is compressed to between 2 and 8 bar. The recycle fluorochemical recovery step comprises a less volatile component removal column  34  and a more volatile component removal column  35 . The less volatile component removal column  34  removes components in the distillation feed gas  33  that are less volatile, e.g. have lower vapor to liquid molar fraction ratios, than the less reactive fluorochemical gas  1 . The less volatile component removal column  34  is a conventional cryogenic distillation column with a reboiler  36 , knock-back condenser  37 , rectifying section  38  and stripping section  39 . Structured packing is preferably used to form the rectifying section  38  and stripping section  39 , but conventional random packing or distillation trays could also be used. The reboiler  36  or knock-back condenser  37  heat duty is adjusted until an acceptable heavy component purge stream  40  is achieved and no freezing occurs in the less volatile component removal column  34  at the selected distillation feed compressor  32  outlet pressure. While either the reboiler  36  or knock-back condenser  37  heat duty is being adjusted to achieve the above product purity and column operability goals, the other duty is adjusted to maintain constant liquid inventory in the less volatile component removal column  34 . The required reboiler  36  and knock-back condenser  37  heat duties and dimensions of the rectifying section  38 , stripping section  39 , reboiler  36  and knock-back condenser  37  can be estimated using standard techniques. The intermediate distillation stream  41  is the overhead product from the less volatile component removal column  34  and the feed to the more volatile component removal column  35 . The more volatile component removal column  35  is also equipped with a knock-back condenser  42 , upper rectifying section  43 , lower rectifying section  44 , stripping section  45  and reboiler  46 . The knock-back condenser  42  is preferably operated at a temperature that is slightly greater than the NF 3  freezing point, e.g. about 66° K, to minimize the NF 3  loss in the light component purge stream  47 . Typically, there is sufficient NF 3  in the intermediate distillation stream  41  to control the temperature and NF 3  inventory in the upper rectifying section  43  using the withdrawal rate of the crude NF 3  product  48  which may be used directly or purified before recycling to the process chamber  3 . If there is insufficient NF 3  in the intermediate distillation stream  41 , then the cryogenic distillation NF 3  feed  49  can be used to control the temperature and NF 3  inventory in the upper rectifying section  43 . Structure packing is preferably used for the upper rectifying section  43 , lower rectifying section  44  and stripping section  45  of the more volatile component removal column  35 . After setting the knock-back condenser  42  temperature, net crude NF 3  product  48  and cryogenic distillation NF 3  feed  49 , the reboiler  46  heat duty is set to achieve the desired purity of the recycle less reactive fluorochemical gas  1 . Conventional techniques can be used to establish reasonable dimensions for the knock-back condenser  42 , upper rectifying section  43 , lower rectifying section  44 , stripping section  45  and reboiler  46 . If the elemental sulfur  22  feed exceeds process requirements, a SF 6 -rich by-product fluorochemical gas  2  is produced. Valuable helium is concentrated in the light component purge stream  47  and may also be advantageously recovered.  
       EXAMPLE 2  
       [0031]      FIG. 3  will be used for the second illustrative example for a chamber cleaning step. In this example, the supplemental gas  4  is substantially pure O 2  and the recycle less reactive fluorochemical gas  1  primary component is SF 6 . A conventional remote plasma produces predominately a F, F 2 , O, SO x F y , and SF 6  chamber cleaning gas to remove debris from the process chamber  3  walls. The expected process chamber  3  off-gas  5  major components would include SF 6 , F 2 , O 2 , SiF 4 , HF, SOF, SOF 2 , and SO 2 F 2  in this case. Therefore, F 2  is the more reactive fluorochemical, SF 6  is the less reactive fluorochemical for recycle, and SO 2 F 2  a less reactive waste product. Stream  5  is heated to about 500° C. to convert substantially all the SO x F y  and SF z  (z&lt;6) species to SF 6  and SO 2 F 2 . Then, the resulting stream  8  is contacted in a conventional aqueous scrubber  9  with an aqueous stream  9  to remove any residual F 2 , SiF 4 , and a substantial portion of the SO x F y  species by hydrolysis via stream  11 . Then, the SF 6  purification system  13  would first remove water from stream  11  using conventional technology and then produce the SF 6  recycle stream  1  and discard stream  15  by conventional distillation.  
         [0032]     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 examples, 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.