Method for removal of moisture from gaseous hydrogen chloride

Abstract: An adsorbent and method for removing water from gaseous HCl. MgCl 2 supported on an activated carbon or silica gel substrate activated by heating to a temperature between 150°C (302°F) and 300°C (572°F) under vacuum will remove water at partial pressures of below 0.5 torr. Activation of the MgCl 2 supported adsorbent can also be effected by heating the adsorbent between 270°C (518°F) and 400°C (752°F) under nitrogen.

Description: BACKGROUND OF THE INVENTION The present invention pertains to removal of moisture or water vapor from gaseous hydrogen chloride (HCl). Gaseous hydrogen chloride is one of the key specialty gases used in the manufacture of semi-conductor devices. HCl is used to grow silicon crystals (epitaxy) and is also used to etch silicon wafers in a dry etching process. For both of these applications high purity HCl is required. High purity HCl is generally taken to mean HCl free of water, or containing less than 0.5ppm of water. HCl containing water is highly corrosive and will present problems to the gas delivery system as well as adversely affecting production yields during the manufacture of semi-conductor devices. For example water contained in hydrogen chloride gas will necessitate frequent replacement of piping, manifolds, valves and the like which are used to deliver the hydrogen chloride to the point of use in the semi-conductor manufacturing process. In cleaning susceptors which are the support structure on which wafers are processed, water contained in gaseous hydrogen chloride will result in a formation of a new oxide on the susceptor thus, frustrating the cleaning function of the hydrogen chloride. In etching applications, water contained in the hydrogen chloride becomes a source of undesirable contamination in a semi-conductor manufacturing environment, which may render chip products made in this environment less than efficient or totally useless for their intended purpose. U.S. Patents 4,853,148 and 4,925,646 disclose a number of adsorptive compositions and methods for removing water from HCI. Among the materials and processes suggested is adsorption using magnesium chloride (MgCl 2 ) supported on an alumina (Al 2 O 3 ) substrate. However, it has been found that the use of MgCl 2 on Al 2 O 3 for moisture removal from hydrogen chloride results in hydrogen chloride slowly reacting with the Al 2 O 3 support to produce a volatile product, AlCl 3 . The AlCl 3 will solidify at cool spots in the downstream process equipment resulting in deposits on the process equipment, such as regulators, filters and valves. This in turn causes system maintenance problems and eventually contamination of the HCI product. Japanese Patent Application H5-13695 discloses removal of water from gaseous hydrogen chloride using a synthetic mordenite. However, it was found that the synthetic mordenite is not stable in gaseous hydrogen chloride. Russian Patent 1726368 discloses removing water and oxygen from hydrogen chloride gas using iron chloride (FeCl 2 ) supported on a carbon substrate. Japanese patent JP 57 015 840 discloses magnesium halide deposited on a porous body, e.g. active carbon, for drying air or other gases. Prior art workers have also used silica gel to remove water from gaseous hydrogen chloride. However, although silica gel and FeCl 2 on carbon are stable in hydrogen chloride their water adsorption capacities are relatively low so that these materials must be changed more frequently when used in a HCl purification system. BRIEF SUMMARY OF THE INVENTION The present invention results from the discovery that an adsorbent consisting of between 5 and 15 percent by weight MgCl 2 on an activated carbon support can be used to remove water from gaseous hydrogen chloride, especially at partial pressures of water in the HCI to be purified in the range of 330 to 667 Pa (0.01 to 0.5 torr). The supported MgCl 2 is activated at a temperature between 150°C and 300°C in vacuum. A preferred method of the invention uses 10 percent MgCl 2 on an activated carbon support. The invention thus is a method of removing water from gaseous HCl wherein the water in the HCl is at a partial pressure of between 3.99 to 66.7 Pa (0.03 to 0.5 torr) by passing the HCl stream containing water over an adsorbent consisting of between 5 to 15% by weight MgCl 2 supported on an activated carbon substrate, wherein the adsorbent is activated at a temperature between 150°C and 300°C under vacuum. BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS Figure 1 is a plot of water capacity against water partial pressure for selected metal chlorides. Figure 2 is a plot of water capacity against water pressure for several adsorbent compositions according to the invention. Figure 3 is a plot of water capacity against water pressure for a preferred composition of the invention supported on activated carbon and silica gel for comparison. Figure 4 is a plot of capacity against water pressure for a preferred composition according to the invention activated at 200°C (392°F) in vacuum, compared to silica gel. 1 Torr = and 133,3 Pa DETAILED DESCRIPTION OF THE INVENTION In order to utilize hydrogen chloride gas in the manufacture of semi-conductor devices it is essential that the hydrogen chloride contain low amounts of retained water vapor or moisture, i.e. less than 0.5 ppm water. Currently manufacturers are using MgCl 2 on an Al 2 O 3 support, synthetic mordenite, silica gel, and FeCl 2 on activated carbon to try and obtain gaseous hydrogen chloride with low water content. It has been discovered that both the MgCl 2 on an Al 2 O 3 material support and mordenite are not stable in HCl. HCl can react with the Al 2 O 3 (also present in mordenite) to form volatile AlCl 3 , which will deposit on downstream components of the process equipment and cause operational problems for the semi-conductor manufacturer. Although silica gel and FeCl 2 on carbon are stable in an HCl environment found in semi-conductor manufacturing processes, the water adsorption capacities are relatively low so that a more frequent change of the purifier material is necessary. According to the present invention adsorbents that are effective to remove water to the desired level from hydrogen chloride gas must be contained on a support that will be resistant to attack by the HCI and be capable of supporting a metal chloride. As a first step MgCl 2 , CaCl 2 , LiCl and FeCl 2 were selected as potential candidates for the metal chloride. Figure 1 illustrates the water capacities for these chlorides at water partial pressure up to 400 Pa (3 torr). Of these MgCl 2 was selected as having the highest water capacity especially at water pressures below 40.0 Pa (0.3 torr) and thus was considered for deposition on a carbon substrate and a silica gel substrate, both known to be resistant to attack by HCl. A number of materials were screened for use as a substrate by testing in a liquid HCl solution and pure water. Each candidate was tested by dropping a sample in a solution of 38% HCl in water and a bath of pure water in order to distinguish reactions caused by HCI from reactions caused by water. The HCl solution and water bath with the samples were maintained at 25°C for 24 hours to observe if there was a color change, break or dissolution. Table 1 summarizes the results of the screening. Material Stability in 38% HCl solution Stability in H 2 O Screening silicalite stable stable pass mordenite not stable stable fail silica gel not stable not stable pass 3A CMS stable stable pass calcium chabazite not stable stable fail 3A zeolite not stable stable fail 4A zeolite not stable stable fail K 2 CO 3 on A1 2 O 3 not stable stable fail CaO not stable relative stable fail Potassium X zeolite not stable stable fail Alcoa Selexsorb not stable stable fail ZSM-5 not stable stable fail AW-500 not stable stable fail The silicalite, silica gel, and carbon molecular sieve samples were all judged to be acceptable for supporting the MgCl 2 even though the silica gel by itself was not stable in either the 38% HCI solution or pure water over the test period. The instability of the silica gel is caused by the swelling effect of the material in the presence of water and not caused by reaction with HCl. Several compositions were prepared by depositing MgCl 2 on activated carbon and silica gel substrates. The solid support was selected to have a high surface area. In the context of the present invention the surface area should broadly be within the range of 300 to 2500 m 2 /g and narrowly within the range of 700 to 1200 m 2 /g. The activated carbon was obtained from Calgon Corp. and identified as type PSC. The silica gel was obtained from Davison Chemical Company and identified as Grade 55. MgCl 2 was deposited or placed on the substrate by the well known incipient wetness technique. As shown in Figure 2, between 5 and 15 weight percent MgCl 2 deposited on a carbon substrate results in an effective adsorbent for removal of water from gaseous HCl, especially at water partial pressures in the HCl of from 1.33 to 66.7 Pa (0.01 to 0.5 torr). The composition of the present invention shows the highest water capacity in the range of water pressure for 3.99 to 39.9 Pa (0.03 to 0.3 torr). Figure 3 is a plot of water capacity in m mol/g against water pressure (torr) for an adsorbent consisting of 10% MgCl 2 supported on a carbon and a comparative silica gel substrate. These are effective compositions for removing the water from the gaseous hydrogen chloride, especially at water partial pressures of between 4.266 to 42.66 Pa (0.032 and 0.32 torr). The carbon supported MgCl 2 material shows a higher water capacity, as shown in Fig. 3. The materials used to derive the data plotted in Figures 2 and 3 were activated at 100°C (212°F) under vacuum. The graphs show the preferred MgCl 2 loading and the preferred solid support according to the invention. Additional studies were conducted to determine the preferred activation temperature. The best adsorption isotherm was obtained for 10% MgCl 2 on activated carbon activated at 200°C (392°F) in vacuum. Figure 4 examplifying the invention is a plot of water capacity (m mol/g) against water pressure (torr) for the preferred composition of the invention activated at a temperature 200°C (392°F) under vacuum compared to silica gel as an adsorbent. Over the range of water partial pressures of 4.266 to 42.66 Pa (0.032 to 0.32 torr) the preferred composition (MgCl 2 on carbon) according to the invention is more effective than silica gel for removal of water from gaseous HCl. It has also been determined by Thermal Gravimetric Analysis that unsupported MgCl 2 could be activated by heating in nitrogen to temperatures between 270°C (518°F) and 400°C (752°F).

Claims: A method of removing water from gaseous HCl wherein the water contained in the HCl is at a partial pressure of between 1.33 to 66.7 Pa (0.01 to 0.5 torr) by passing the HCl stream containing water over an adsorbent consisting of between 5 to 15% by weight MgCl 2 supported on an activated carbon substrate, wherein the adsorbent is activated by heating to a temperature between 150°C and 300°C under vacuum prior to passing said HCl stream containing water over said adsorbent. A method according to claim 1 wherein 10% by weight MgCl 2 is present on said activated carbon.

Inventors: DONG CHUN CHRISTINE, GOLDEN TIMOTHY CHRISTOPHER, HSIUNG THOMAS, DONG, CHUN CHRISTINE, GOLDEN, TIMOTHY CHRISTOPHER, HSIUNG, THOMAS, HSIUNG, THOMAS HSIAO-LING
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1998
Main classification: B01J20
IPC codes: B01D53, B01J20, C01B7