Patent Application
20090068086
The present invention relates to new and useful methods and apparatus for the production of high purity tungsten hexafluoride (WF6) and more particularly to the use of a carbonaceous material to remove impurities from WF6. The present invention also relates to activated carbonaceous material for use in the purification of WF6 and methods of making the activated carbonaceous material.
Tungsten hexafluoride (WF6) a useful reagent for the production of very large scale integration (VLSI) semiconductor devices, particularly dynamic random access memory (DRAM) and high performance microprocessors. WF6 is typically used in chemical vapor deposition (CVD) and atomic layer deposition (ALD) unit operations to produce tungsten contact plugs and tungsten silicide electrodes and in addition, WF6 reacts with aluminum and may be used to produce aluminum trifluoride studs for semiconductor circuits. The WF6 gas used for these purpose must be very pure and free of contaminants to avoid problems with the deposited layers. In particular, typical maximum gaseous impurity levels for these applications are 1 parts per million (ppm) N2, 1 ppm O2+Ar, 0.5 ppm CO, 1 ppm CO2, 0.5 ppm SiF4, 0.5 ppm SF6, 1 ppm CF4, and 10 ppm HF. Typical maximum liquid phase impurities required by the electronics industry are 10 parts per billion (ppb) Al, 10 ppb As, 10 ppb B, 16 ppb Ca, 2 ppb Cd, 10 ppb Cr, 20 ppb Cu, 10 ppb Fe, 10 ppb K, 10 ppb Mn, 10 ppb Na, 10 ppb Mg, 25 ppb Mo, 100 ppb Ni, 0.05 ppb U, and 0.05 ppb Th.
WF6 gas is usually produced by the reaction of gaseous F2 with a high purity tungsten powder at a temperature greater than about 350° C. As a result of the high heat of reaction (≈−1721.72 KJ/Gm Mole), the gaseous fluorine feed is typically diluted, such as with recycled WF6 product (see U.S. Pat. Nos. 5,328,668 and 5,348,723) or with nitrogen. Use of recycled WF6 requires a complex gas recycle system, while the use of nitrogen requires subsequent separation of the WF6 product from the nitrogen diluent. Prior art methods to remove high volatility impurities generally employ a condenser arrangement, wherein the condenser temperature must be substantially below the WF6 freezing point and therefore frozen WF6 must be periodically removed from the cooling surfaces by desublimation (see U.S. Pat. No. 5,324,498). This requires periodic discontinuation of the feed gas and additional equipment to heat the vessel walls and recover the frozen WF6 product. Another disadvantage of this operation is that a significant fraction of the WF6 particles are very small and do not readily collect on the cooling surface, thereby reducing product yield.
Because of impurities in the starting tungsten metal materials, the product WF6 will include impurities. While the level of impurities in the WF6 product can be reduced by purifying the starting tungsten metal material, some level of impurities will inevitable be present and end up in the WF6 product. Therefore, the WF6 will require further purification in order to meet the required specifications noted above. There have been numerous proposals for purifying WF6, primarily falling into two categories; adsorption techniques (see U.S. Pat. No. 5,234,679 and US Published Application 2003/0091498 for MoF6 removal; Russian Patent SU 1787937 for CrO2F2 removal; Japanese Patent JP 2124723 for HF removal; and JP 11-180716 for Cr compound removal) and distillation techniques (see European Patent 1070680). However, none of the prior art provides adequate removal of all of the impurities, particularly of molybdenum and chromium impurities. Distillation techniques have not proved to be adequate because of the close melting and boiling points of WF6 and molybdenum and chromium compound impurities. In addition, the prior art adsorption proposals have been unable to meet the requirements needed by the electronics industry.
More recently, the prior art has explored the production of carbonaceous materials (See US Patent Application 2004/0084793; and U.S. Pat. No. 3,674,432) and the use of such materials for the purification of fluorinated compounds (see U.S. Pat. No. 6,955,707). However, none of this prior art is directed at the purification of WF6.
There remains a need in the art for improvements to purifying WF6 gas, particularly for use in the electronics or semiconductor industry and particularly for the removal of molybdenum and chromium impurities. There also remains a need in the art for carbonaceous materials that are useful in the purification of WF6 gas.
The present invention provides new and useful apparatus and methods for purifying WF6 gas, and in particular, provides apparatus and methods of using carbonaceous materials to produce high purity WF6 by removing substantially all the high volatility gas impurities and troublesome transition metal impurities. The present invention is particularly useful for removing chromium and molybdenum impurities from WF6 gas.
The present invention provides new and useful apparatus and methods for purifying WF6 gas, and in particular, provides apparatus and methods of using carbonaceous materials to produce high purity WF6 by removing the high volatility gas impurities and troublesome transition metal impurities. The present invention is particularly useful for removing chromium and molybdenum impurities from WF6 gas.
The present invention will be described with reference to the drawings figures, wherein
The WF6 synthesis reactor stage 1 includes a WF6 reactor 10, in which an initial WF6 product gas 19 is produced by contacting tungsten containing feed 15 with fluorine containing feed 16. Tungsten containing feed 15 can be either a high purity tungsten powder or a high purity tungsten compound, such as WO3. It is important to select tungsten containing feed 15 to have low levels of transition metal impurities, e.g. transition metals from Group IIIB (including the Lanthanide and Actinide series), IVB, VB, VIB, VIIB, VIII, IB, and IIIB of the periodic table of elements. It is particularly important to select tungsten containing feed 15 to have low levels of chromium and molybdenum. Fluorine containing feed 16 is preferably F2 gas. Reactor 10 typically operates in the 350° C. to 600° C. temperature range, with the reaction temperature controlled by a combination of cooling through the reactor wall or the optional addition of gaseous diluent 17 to fluorine containing feed 16. Gaseous diluent 17 may be any inert gas, for example, N2, Ar, NF3 or WF6. The reaction pressure is slightly greater than atmospheric pressure, e.g. in the range of 110 kPa to 210 kPa. Tungsten containing feed 15, fluorine containing feed 16 and optional gaseous diluent 17, react within reactor 10 to produce reactor product gas 19. Reactor 10 can be any standard reactor such as a fixed bed or an expanded bed. An expanded bed reactor is preferred because of superior heat transfer and solid-vapor contacting characteristics. However, entrainment of tungsten containing particles in product gas 19 is an inherent disadvantage of using an expanded bed reactor. This disadvantage can be minimized by using a spouted bed reactor or by using a cyclone to recover the tungsten containing particles from product gas 19 and recycling such to reactor 10. The conversion of tungsten containing feed 15 to product gas 19 is very high, but some solid reaction products will eventually accumulate on the walls of reactor 10 and therefore will need to be periodically removed as solid by-product 18.
Product gas 19 moves on to crude WF6 recovery stage 2 that will be described with reference to both
Crude WF6 recovery stage 2 uses a temperature swing adsorption (TSA) or pressure swing adsorption (PSA) process to remove high volatility impurities from product gas 119. The operation of a TSA process comprising a three step cycle will be described in more detail. While the cycle will be described with reference to a single adsorption vessel 20 shown in
T
9,min=3088/15.25−ln [Y9,WF6P9] Equation 1
wherein T9,min is the minimum product gas 19 temperature (° K), Y9,WF6 is the molar fraction of WF6 in product gas 19, and P9 is product gas 19 total pressure (kPa). As noted above the reaction pressure for product gas 19 is between 110 kPa and 210 kPa. The molar fraction of WF6 in product gas 19 is between 0.1 and 0.8, and preferably between 0.2 and 0.7.
Carbonaceous material 21 separates WF6 to be separated from high volatility impurities at the operating temperature. This is because high volatility impurities, such as N2, have a boiling point that is less than the WF6 freezing point of 275.5° K. Therefore, while WF6 is adsorbed onto carbonaceous material 21 at a temperature just above the estimated operating temperature from Equation 1, the high volatility impurities continue through adsorption vessel 20. The preferred first step operating temperature is preferably less than 20° K, and more preferably less than 10° K, greater than the minimum temperature estimated using Equation 1. The operating pressure of the first step is preferably equivalent to product gas 19 pressure as determined by reactor 10 operation, (e.g. 110 kPa to 210 kPa), but may be increased using an optional compressor.
In the configuration shown in
1. Umf=[μ/Pf(1/Σ1/xidpi)]{[1135.7+0.0408((1/Σ1/xidpi)3Pf(Pi−Pf) g/μ2)]0.5−33.7} Equation 2
wherein μ is the gas viscosity, Pf is the gas phase density, dpi is the hydraulic diameter for particle size range i, xi is the mass fraction particle size range i, Pi is the particle density, and g is the local acceleration of gravity. The particle hydraulic diameter is equivalent to six times the particle volume to area ratio. The superficial gas velocity through carbonaceous material 21 is preferably less than 75%, and more preferably less than 50%, of the minimum fluidizing velocity estimated by Equation 2. The superficial gas velocity in crude WF6 recovery stage 2 is also preferably greater than 1.5 times, and more preferably greater than 3 times, the minimum gas superficial velocity in reactor 10, in order to minimize accumulation of particles from product gas 19 in carbonaceous material 21. A down-flow configuration can also be used, but is more susceptible to bed compression and excessive pressure drop from accumulation of small particles at the top of the bed.
Product gas 19 enters adsorption vessel 20 through valve 23, passes through carbonaceous material 21 where WF6 is adsorbed and high volatility gases exit through off-gas valve 25 as waste stream 26. This flow continues until commercially significant quantities of WF6 are observed in waste stream 26 signaling saturation of carbonaceous material 21, at which point the first step of the cycle is completed by closing valve 23 so that product gas 19 is directed to a further adsorption vessel, equivalent to adsorption vessel 20 through header line 27. Waste stream 26 may be treated by a conventional aqueous caustic scrubber to remove toxic impurities, such as elemental fluorine and fluorinated products and then discharged to the atmosphere or otherwise disposed.
In the second step of the cycle for crude WF6 recovery stage 2, WF6 is desorbed from carbonaceous material 21 by increasing the temperature of the bed. The second step begins by closing valves 23 and 25 and then increasing the temperature of carbonaceous material 21. This may be done by direct heating 210 through adsorption vessel 20 wall, but because of the low thermal conductivity of carbonaceous material 21, there is a significant limit to the efficiency of such a direct conduction heating method. Therefore, it is preferable according to the present invention to use an indirect heating method, such as using a heat pump comprising condensing leg 220 and boiling leg 225. Heat 230 is withdrawn from condensing leg 220 in order to maintain condensing leg 220 temperature between the WF6 dew point and freezing point temperatures. As a result, liquid WF6 accumulates in condensing leg 220 and then flows to boiling leg 225. Heat 235 is added to boiling leg 225 and circulating flow of gaseous WF6 through carbonaceous material 21 is initiated by opening heat pump inlet valve 240 and heat pump outlet valve 245. The temperature of the gaseous WF6 flowing through valve 245 is preferably between 325° K and 500° K and more preferably between 350° K and 475° K. The increasing temperature of carbonaceous material 21 results in WF6 desorption from carbonaceous material 21, thereby increasing pressure in adsorption vessel 20 as well as the heat pump, resulting in increased circulating flow of the gaseous WF6. The pressure is allowed to increase to between 125 kPa and 300 kPa and preferably between 150 kPa and 300 kPa by controlling the flow rate of crude WF6 stream 22 through valve 28. Heating of carbonaceous material 21 continues until the difference in the WF6 gas temperature at valves 240 and 245 is less than 40° K, and preferably less than 20° K, at which time the second step of the cycle is completed and the third step can begin.
In the third step of crude WF6 recovery stage 2, carbonaceous material 21 is cooled. In one embodiment of the present invention, cooled purge gas 250, preferably N2 is used to cool carbonaceous material 21 and to remove WF6 from the interstitial volume of carbonaceous material 21. Purge gas 250 may be cooled by conduction prior to entering adsorption vessel 20 through valve 252 and by joule-Thompson effect cooling by adiabatic expansion across valve 252 when vacuum pump 29 is operating. In addition, carbonaceous material 21 may be cooled by heat conduction 215 through adsorption vessel 20 walls, but once again the low thermal conductivity of carbonaceous material 21 significantly and adversely affects the cooling efficiency of such a direct conduction method. It is advantageous to use vacuum pump 29 to aid in the desorption of the WF6 from the interstitial volume of carbonaceous material 21 and to cool carbonaceous material 21 via flow through valve 254. Purge gas 250 may also be fed directly to vacuum pump 29 through valve 256. Cooling of carbonaceous material 21 continues until adsorption vessel 20 pressure is less than 50 kPa and preferably less than 10 kPa, at which point the third step of the cycle is complete and a new cycle can begin.
Over time, the properties of carbonaceous material 21 in adsorption vessel 20 degrades for a number of reasons, including accumulation of particles from product gas 19, such as previously entrained tungsten. This particle accumulation causes maldistribution of product gas 19 through carbonaceous material 21 and eventually causes excessive pressure drop across carbonaceous material 21. Purge gas 250 can be used to periodically remove the accumulated particles from carbonaceous material 21 by flow through valves 260 and 25 at a sufficient rate to expand carbonaceous material 21. The entrained particles may be recycled to reactor 10 via stream 265 that can feed into tungsten containing stream 5. Product gas 19 typically contains between 25 ppm and 100 ppm of hydrogen fluoride that acts to catalyze the intercalation reactions of metal and non-metal fluorides with carbonaceous material 21 and facilitates the removal thereof. Product gas 19 also typically contains between 1 ppm and 10,000 ppm elemental fluorine depending on the efficiency in reactor 10. This elemental fluorine can increase the fluorine feed requirement, increase waste handling requirements, and cause faster degradation of carbonaceous material 21. To avoid these problems, the elemental fluorine content of product gas 19 is kept to less than 1,000 ppm. Ultimately, carbonaceous material 21 will need to be periodically removed as carbonaceous material by-product 270 and replaced with fresh carbonaceous material 200 to maintain the performance of adsorption vessel 20. Carbonaceous material 200 is prepared according to the methods of the present invention as discussed in detail below.
The above description is of a TSA process for removing high volatility impurities from WF6. As noted, it is also possible to use a PSA process as will be recognized by those skilled in the art.
Crude WF6 stream 22 contains very low levels of high volatility impurities but still has unacceptable levels of transition metals, such as chromium and molybdenum. Therefore, in accordance with the present invention, crude WF6 stream is processed further in WF6 purification stage 3 that will be described with reference to
Crude WF6 stream 22 may be treated as in either a gas phase or a gas-liquid phase. As crude WF6 stream 22 flows through purification column 30, transition metals are adsorbed into carbonaceous material 31 and purified WF6 stream 33 is produced. The feed flow through WF6 purification column 30 and carbonaceous material 31 may be either a downflow configuration, as shown in
The above describes the purification of crude WF6 stream 22. However, WF6 purification stage 3 can also be used to purify WF6 from other sources, such as purchased WF6 stream 34 or recycled WF6 distillation product 44 from distillation stage 4. WF6 purification stage 3 can be carried out using any of a liquid phase feed, a gas phase feed or a gas-liquid phase feed to purification column 30. When the feed is in liquid form, purification column 30 may be operated in a trickle bed mode as shown in
The operation of WF6 purification stage 3 as shown in either
In the description above, the WF6 purification stage 3 follows a crude WF6 recovery stage 2. However, if the starting WF6 gas has sufficiently low levels of high volatility impurities, the purification can comprise only the WF6 purification stage 3 to remove transition metal impurities, particularly molybdenum and chromium.
Purified WF6 stream 33 may be further treated in distillation stage 4 that will be described with reference to both
The present invention is very effective at removing both high volatility impurities and transition metal impurities, particularly chromium and molybdenum, as will be shown more fully in Example 1. Control of impurity levels in WF6 product 45 from distillation stage 4 can be accomplished by treating the recycled WF6 distillation product and blending a portion of purified WF6 stream 33 with WF6 product 45 to achieve the desired quality.
The present invention also relates to the activation and conditioning of carbonaceous material for use in the purification of WF6 and to the carbonaceous material produced. Commercially available activated carbon adsorbents or custom materials may be used in granular or shaped form. Preparation of custom carbonaceous materials begins by selecting the carbonaceous material precursor that can be any of coal, wood, nut shells, peat, coal or petroleum pitch, or coal or petroleum coke. The carbonaceous precursor material is combined with a binder, e.g. Teflon, petroleum or coal tar pitch, to increase the physical strength, and a suitable solvent to provide access to the internal surface area of the carbonaceous material. This blend can then be formed into pressed briquettes or extruded pellets to produce a shaped carbonaceous material. The shaped carbonaceous material is thermally activated by contact with combustion gases at a temperature between 400° C. and 3500° C. and preferably between 500° C. and 1500° C. Use of steam during the thermal activation serves to increase surface area. If using commercially available activated carbon adsorbents, a reactivation process is preferably used to remove adsorbed hydrocarbons and water. This reactivation process comprises heating the carbon adsorbent with combustion gases or in an inert atmosphere, preferably N2, to a temperature between 100° C. and 1000° C. and preferably between 200° C. and 600° C.
The activated carbonaceous material is conditioned according to the present invention by treatment with a fluorination agent. The following list summarizes the relative fluorination activity for a variety of fluorination agents: OF2>F2>NF3>ClF3>BrF2>IF7>CuF2>IF5>SF6>MnF4>CF4>AsF5>MoF6>CrF5>WF6>FeF3>NiF2>UF6>MgF2>BF3>AlF3>ThF4>CaF2. The preferred fluorination agent has a fluorination activity and temperature that are greater than or equal to the fluorination activity and temperature of the major fluorine containing compound that contacts the carbonaceous material in crude WF6 recovery stage or WF6 purification stage described above. For example, in the present invention and with reference to the drawing figures, product gas 19 feed to crude WF6 recovery stage 2 could contain significant F2 partial pressures, particularly in the event that the tungsten inventory in reactor 10 decreases significantly. Therefore, fresh carbonaceous material 200 should be conditioned at a temperature and F2 partial pressure that is greater than the maximum anticipated values in product gas 19. However, higher activation temperatures and fluorination agent fluorine activity increase the fluorine to carbon molar ratio and decrease the reactivity of the carbonaceous material with respect to transition metal fluorides. Therefore, a fluorination agent with a lower fluorine activity is desirable for conditioning fresh carbonaceous material 300 used in WF6 purification stage 3 than that used for crude WF6 recovery stage 2. In the present invention, because the primary goal of WF6 purification step is the removal of molybdenum and chromium impurities, WF6 is the preferred fluorination agent.
In a particular embodiment of the present invention, carbonaceous material 31 adsorbs transition metal impurities during WF6 purification stage 3. This adsorption of transition metal is very limited and essentially irreversible. Therefore, it is advantageous to condition spent carbonaceous material 38 from WF6 purification stage 3 using the conditioning procedures described above and then use the resulting material as fresh carbonaceous material 200 for crude WF6 recovery stage 2.
The use of carbonaceous materials to purify WF6 according to present invention provides several advantages. In particular, prior art distillation systems can not efficiently recover low concentrations of WF6 from high volatility diluents, such as nitrogen, without operating below the WF6 freezing temperature. The present invention overcomes this problem because the high volatility species can be removed much more efficiently in crude WF6 recovery stage 2 using adsorption on carbonaceous material. In addition, some transition metal compounds, particularly chromium and molybdenum compounds, are difficult to separate from WF6 by distillation. In accordance with the present invention, chromium and molybdenum compounds can be much more efficiently removed by contacting WF6 with carbonaceous material in WF6 purification stage 2. In particular, by using the present invention, chromium compound impurities can be removed to levels well below 25 ppb, preferably less than 10 ppb and more preferably less than 1 ppb. Similarly, molybdenum compound impurities can be removed to levels well below 10 ppb, preferably less than 5 ppb and more preferably less than 1 ppb.
This example illustrates the use of a carbonaceous material according to the present invention to remove less volatile transition metal species from a WF6 stream having essentially no elemental fluorine. Tungsten hexafluoride feed material purified using a conventional distillation process was purchased and supplied in cylinders. The average molybdenum impurity level of 60 parts per billion by weight (ppb) was substantially greater than the 25 ppb molybdenum impurity specification required by the electronics industry. Similarly, the average chromium impurity level of 41 ppb was substantially greater than the 10 ppb chromium impurity specification required by the electronics industry. Attempts to use standard distillation techniques to decrease these transition metal impurity levels were unsuccessful despite a significant decrease in the WF6 yield. A carbonaceous material was produced according to the present invention by purchasing a commercially available shaped activated carbon (NORIT® RX3 Extra from NORIT America, Inc.) and conditioning this carbonaceous material in a WF6 atmosphere at 50° C. until essentially no carbon tetrafluoride was observed in the product gas. The WF6 product was then contacted with the carbonaceous material at 50° C. with a 1 second space time according to the process of the present invention. The data in Table 1 shows that the carbonaceous material removed more that 99% of the molybdenum impurities and more than 98% of the chromium impurities on average. The resulting average molybdenum impurity level of 0.11 ppb was substantially below the required standard of 25 ppb, and the average chromium impurity level of 0.69 ppb was substantially below the required standard of 10 ppb.
This example illustrates the use of a carbonaceous material according to the present invention to remove high volatility components from a dilute WF6 in nitrogen stream with potential for a high F2 partial pressure of 25 kPa. A high fluorine content carbonaceous material is compatible with the potentially high fluorine partial pressure and therefore a high fluorine content CFx powder (Advance Research Chemical's (ARC) Carbofluor™ CFx with an x value of about 1.15) was selected as the starting carbonaceous material. A mixture of 43 weight percent adsorbent carbonaceous material (ARC Carbofluom™ grade 2065 powder), 5 weight percent fluorine resistant binder (Dyneon TFTM 2071 polytetrafluoroethylene (PTFE) powder), and 52 weight percent solvent (3M FluorinertTM FC-84 solvent) to prevent binder blockage of the CFx pores, was blended. The blended mixture was extruded using a Amandus Kahl laboratory L175 pellet press with a 3 millimeter die and the resulting pellets were baked at about 100° C. to remove the solvent. The adsorption characteristics of this carbonaceous material were determined over a range of operating conditions for a number of fluorinated species.
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.
