Patent Publication Number: US-8535423-B2

Title: Removal of impurities from hydrogen-containing materials

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 12/466, 272, filed May 14, 2009, now U.S. Pat. No. 8,268,046 B2, which claims priority to U.S. Provisional App. No. 61/053,990, filed May 16, 2008, and titled “REMOVAL OF IMPURITIES FROM METAL HYDRIDES”, the entire contents of which are herein incorporated by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     A wide range of hydride gases are used to make materials for semiconductor devices. As these devices continue to increase in complexity and precision, the purity requirements for the starting materials that are used to make them has also increased. 
     Metal hydrides such as silanes (i.e., hydride gases of silicon) are used in a variety of semiconductor fabrication processes, including epitaxial growth of silicon metal films to make silicon wafer substrates. In vapor-phase epitaxy (VPE) gases of silicon halides and/or silicon hydrides (silanes) are chemically decomposed on a substrate surface to deposit silicon metal and vapor-phased exhaust products such as hydrogen gas (H 2 ) and hydrogen halides (e.g., HCl(g) when the silicon precursor is a silicon chloride). 
     During the growth of the silicon layer by VPE, moisture levels are carefully monitored and controlled. Unlike chemical vapor depositions of silicon dielectric films like silicon oxide, even relatively low levels of moisture contamination is problematic for silicon VPE because the oxidation of a silicon metal film has a much larger adverse impact on the film&#39;s electrical characteristics. Water vapor is one of the most common, and yet most difficult impurities to remove from process gases like silanes. Most drying processes try to reduce the moisture levels in the silane to a parts-per-million (ppm) range. However, silicon VPE processes for making silicon films for modern semiconductor and electronic components more often require reduced moisture levels in the tens to hundreds parts-per-billion (ppb) range. 
     Unfortunately, the delivery of ultra-high purity silane gases with low levels of moisture contaminants faces a number of challenges. Moisture and other contaminants in silanes can come from the fabrication of the source gas itself, as well as during the subsequent packaging, shipment, storage, and handling of the gas. While the silane gas manufacturer typically provides a contaminant analysis when the gas leaves the production facility, these purity levels often change as additional contaminants are introduced from outgassing by the containers (e.g., gas cylinders), as well as leaks and residual contaminants present in downstream delivery equipment. 
     The presence of moisture in the ambient atmosphere provides many opportunities for moisture to contaminate highly pure silanes and other metal hydrides before reaching their point of end use. Thus, there remains a need for methods and materials to purify metal hydrides such as silanes to a point where they are acceptable for use in modern electronic and semiconductor manufacturing processes. This and other problems are addressed by embodiments of the present invention. 
     BRIEF SUMMARY OF THE INVENTION 
     Purifying methods, materials, and systems are described for removing moisture and other impurities from hydrogen-containing gases such as, for example, silane gases. One example of a silane gas used in embodiments of the invention is trisilane (Si 3 H 8 ). Trisilane can be used to grow thin films of pure silicon (Si) and silicon-germanium (Si—Ge) alloys at lower temperature than silane (SiH 4 ), which is sometimes referred to here as “monosilane” to avoid confusion with the term “silanes” that refers to the class of silicon hydride compounds Si x H y  (where x=1 or more, and y=2x+2). 
     While trisilane may offer some advantages over monosilane, especially for meeting low thermal budget requirements, it can be more challenging to purify. Many conventional monosilane purification materials used primarily for moisture removal can also significantly decompose trisilane into smaller silanes like monosilane and disilane. Thus, the purifier materials and activation methods have to be carefully selected to accomplish the moisture removal goals without substantially decomposing poly-metal hydrides (e.g., M x H y  were x=2 or more) into smaller hydrides and/or other decomposition products. 
     Furthermore, poly-metal hydride production methods also typically produce some fraction of higher metal hydrides. For example, the production of trisilane typically produces tetrasilane (e.g., n-tetrasilane and iso-tetrasilane) impurities. Embodiments include methods and materials that can remove (e.g., decompose) these higher metal hydrides at a faster rate than the poly-metal hydride, increasing the relative concentration of the poly-metal hydride in the purified product. 
     Embodiments of the invention include methods of purifying a hydrogen-containing material. The methods may include the steps of providing a purifier material comprising silica. The silica may be heated at temperature of about 100° C. or more in a dry atmosphere to form activated silica. The activated silica may be contacted with the starting hydrogen-containing material, where the activated silica reduces a concentration of one or more impurity from the starting hydrogen-containing material to form the purified hydrogen-containing material, and where the activated silica does not decompose the purified hydrogen-containing material. 
     Embodiments of the invention also include methods of reducing impurities in a poly-metal hydride. The methods may include the steps of providing a purifier material comprising silica and having a SiO 2 :Al 2 O 3  mole ratio greater than 1000. The purifier material may be contacted with the poly-metal hydride, where the purifier material reduces a concentration of higher metal hydrides that have a greater number of metal atoms than the poly-metal hydride. Embodiments may also include having the purifier material reduce the concentration of lower metal hydrides in the poly-metal hydride, wherein the lower metal hydrides have a smaller number of metal atoms than the poly-metal hydride that is being purified. 
     Embodiments of the invention further include systems to generate purified hydrogen-containing material. The systems may include a source of starting hydrogen-containing material comprising one or more impurities. The systems may also include a purifier unit comprising activated silica. The activated silica may be formed by heating the silica to a temperature of about 100° C. or more in a dry atmosphere. The systems may still further include an outlet downstream from the purifier unit to release the purified hydrogen-containing material from the purifier unit, where the purified hydrogen-containing material has a reduced concentration of the one or more impurity. 
     Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the invention. The features and advantages of the invention may be realized and attained by means of the instrumentalities, combinations, and methods described in the specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings wherein like reference numerals are used throughout the several drawings to refer to similar components. In some instances, a sublabel is associated with a reference numeral and follows a hyphen to denote one of multiple similar components. When reference is made to a reference numeral without specification to an existing sublabel, it is intended to refer to all such multiple similar components. 
         FIG. 1  is a flowchart showing selected steps in methods of purifying hydrogen-containing compositions according to embodiments of the invention; 
         FIG. 2  is a flowchart showing selected steps in methods of reducing impurities in hydrogen-containing compositions according to embodiments of the invention; 
         FIG. 3  is a simplified schematic of a system for purifying hydrogen-containing compositions according to embodiments of the invention; 
         FIG. 4  is a schematic for a trisilane purifier material testing system; 
         FIG. 5  is a plot of silanol group density as a function of silica gel activation temperature; 
         FIG. 6  is a graph of the moisture impurity concentration in a trisilane sample; 
         FIG. 7  is a chromatogram of a trisilane sample; and 
         FIG. 8  is a graph of various impurity concentrations to plot the effect of the purifier material on the purity of the trisilane produced. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Methods and systems are described for removing impurities (such as moisture) from hydrogen-containing materials (such as metal hydrides). The purifier materials are selected and treated so that they remove a majority of the impurities without decomposing large amounts of the hydrogen-containing materials into smaller species. For example, when the hydrogen-containing material is a poly-metal hydride, purifier materials are selected and treated to reduce the decomposition of the poly-metal hydride into lower hydrides that contain fewer numbers of metal atoms (e.g., the decomposition of trisilane into mono- and disilanes). 
     The moisture levels in the purified hydrogen-containing materials have a substantially reduced amount of moisture, with reductions in moisture concentration of 90% or more being achievable. Recontamination of the purified hydrogen-containing materials may also be reduced by purifying them in situ at a point of use. The reduced transport, storage time, and exposure to gas delivery equipment can keep the impurity concentrations low as the purified hydrogen-containing materials are immediately consumed in, for example, semiconductor manufacturing equipment. 
     The hydrogen-containing materials that can be purified may include mono-hydrides (e.g., hydrides with a single metal atom) and poly-hydrides that have two or more non-hydrogen elements that are bonded together. For example, poly-hydrides may include poly-metal hydrides that have two or more metal atoms that are bonded together. Poly-metal hydrides may also include isomeric configurations of the hydrides (e.g., n-tetrasilane versus iso-tetrasilane). 
     The hydrogen-containing materials may include a variety of materials, including Group III (e.g., B, Al, Ga, In, Tl), Group IV (e.g., C, Si, Ge, Sn, Pb), and Group V (e.g., N, P, As, Sb, Bi) elements that are bonded to one or more hydrogen atoms. The materials may also include combinations of two or more of these elements. When the hydrogen-containing material is a metal hydrides, the hydride may include any metal element capable of forming at least a partially stable hydride. These may include metals from the periodic table&#39;s Group III (e.g., B, Al, Ga, In, Tl), Group IV (e.g., Si, Ge, Sn, Pb), and/or Group V (e.g., P, As, Sb, Bi), among other metals. The metal hydrides may also include combination of two or more different metal elements (e.g., a hydride comprising Si and Ge, Si and P, Si+P+Ge, etc.). The metal hydride sample may also include combinations of two or more metal hydride compounds, (e.g., trisilane and poly-germanium hydride). 
     Exemplary Methods 
       FIG. 1  shows selected steps in a method  100  of purifying a hydrogen-containing material according to embodiments of the invention. The method  100  may include providing a purifier material  102  which may be a silica (e.g., a silica gel and/or a more crystalline silicate). The purifier material is then heated  104  to activate the material. When the purifier material is a silica, the heating step may involve raising the temperature of the silica in a dry atmosphere (i.e., an atmosphere with a reduced level of water vapor). The heating temperature may be 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, or 600° C. or more. For example, the heating temperature may range from about 150° C. to about 400° C., or from about 400° C. to about 600° C., among other temperature ranges. 
     The dry atmosphere may be made of inert gases, noble gases, reducing gases, and/or combinations of these gases. Inert gases may include dry nitrogen (N 2 ), and noble gases may include helium, argon, krypton, etc. The reducing gases may include hydrogen (H 2 ) and/or ammonia (NH 3 ), among other reducing gases. Embodiments may also include heating the purifier material under vacuum (i.e., a reduced pressure atmosphere). 
     The dry atmosphere may be static or dynamic (i.e., flowing). For example, the gas(es) that make up the dry atmosphere may be continuously flowing through the purifier material for a portion or the entire duration of the heating step (e.g., about 3, 6, 9, 12, 15, 18, 21, 24 hours, etc.). 
     Following the activation of the purifier material, the temperature may be optionally adjusted  106 , for example to room temperature or below (e.g., about 25° C. or less, about 0° C., etc.), before or during purification of the hydrogen-containing material. Alternatively, the purifier can be maintained at the activation temperature, or a temperature between room temperature and the activation temperature. In still other embodiments, the purifier material adjusts to the ambient temperature of the purifier system. When the temperature of the purifier material is adjusted to a temperature that is different than ambient temperature, a cooling or heating system may be placed in thermal contact with the material to maintain its temperature. 
     The activated purifier material may be contacted with the starting hydrogen-containing material  108  to remove one or more impurities from the material. These impurities may include water vapor (i.e., moisture), siloxanes, germoxanes, silyl-germoxanes, and/or hydrocarbons, among other types of impurities. The reduced concentration of water vapor in the purified hydrogen-containing material may be 90%, by wt., or more compared with the starting hydrogen-containing material. For example, the water vapor concentration in a starting trisilane sample (e.g., about 3000-6500 ppb) may be reduced to about 1 ppm or less, about 500 ppb or less, about 300 ppb or less, about 100 ppb or less, about 50 ppb or less, etc., in the purified trisilane that has flowed through the purifier material. The impurities may also include decomposition products of the hydrogen-containing material being purified. For example, when the metal hydride is a poly-silane (e.g., silane having formula Si x H y , wherein x=2 or more, and y=2x+2), decomposition products may include silanols, and silanes having fewer Si atoms (e.g., monosilane). 
     When the hydrogen-containing material is a metal hydride, the impurities may also include higher and/or lower metal hydrides that have a greater and/or smaller number of metal atoms than the desired metal hydride. The higher metal hydrides may be formed by the combining or splitting of the desired metal hydride during synthesis and/or storage. If the desired metal hydride has the chemical formula M x H y  (where x=1 or more, and y=2x+2) the higher metal hydride may have a formula of M x+z H y+2z  (where z=1 or more). For example, when the desired metal hydride is trisilane (Si 3 H 8 ), the higher polysilane impurities may include straight and isomeric forms of tetrasilane, pentasilane, hexasilane, heptasilane, etc. Similarly, a lower metal hydride may have the formula M x−z H y+2z  (where x=2 or more, and z=1 or more) 
     Referring now to  FIG. 2 , a flowchart having selected steps in methods  200  of reducing impurities in hydrogen-containing materials such as metal compositions according to embodiments of the invention is shown. The methods  200  may include the step of selecting a silica purifier material  202  that has a ratio of SiO 2 :Al 2 O 3  (i.e., the silica to alumina ratio) that may be a certain number or more. For example, the SiO 2 :Al 2 O 3  may be about 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, 10000, 11000, 12000, 13000, 14000, or 15000 or more. 
     The threshold ratio may vary for different types of metal hydrides. For example, if the metal hydride is a polysilane like trisilane, the silica to alumina ratio may be about 1000 or more (e.g., about 12000 or more). If the silica to alumina ratio drops below about 1000, the high relative concentration of alumina in the purifier material may decompose too much of the trisilane into lower silanes like mono- and disilane. 
     The selected silica purifier may then be put in contact with the starting metal hydride to be purified  204 . As noted above, the silica purifier has been selected to significantly reduce the rate of decomposition of the metal hydride compared with purifiers having a lower SiO 2 :Al 2 O 3  ratio. Decomposition may involve the splitting of the metal hydride into lower hydrides and/or non-hydride decomposition products. For example, if the chemical formula of the metal hydride is M x H y  (where x=1 or more, and y=2x+2) decomposition products may include lower metal hydrides having a formula of M x−z H y−2z  (where z=1 or more). For example, when the metal hydride is trisilane (Si 3 H 8 ), the decomposition products may include monosilane (SiH 4 ) and disilane (Si 2 H 6 ). 
     Contact between the silica purifier and the starting metal hydride causes impurities to be removed from the metal hydride  206 . As noted above, these impurities may include water vapor, siloxanes, hydrocarbons, etc., among other types of impurities. They may also include higher metal hydrides that have a greater number of metal atoms than the desired metal hydride. 
     The starting metal hydride may be stored premixed with other fluid components (i.e., other gases and/or liquids), or may be mixed prior to contacting (e.g., flowing) through the purifier material. These additional components may include carrier gases and/or reducing gases, and their concentrations and compositions may depend on the final application of the metal hydride. The carrier gases may include inert gases and/or noble gases such as nitrogen (N 2 ), helium, argon, krypton, etc. The reducing gases may include hydrogen (H 2 ) and/or ammonia (NH 3 ), among other reducing gases. 
     Exemplary Systems 
       FIG. 3  shows a simplified schematic of a system  300  for purifying hydrogen-containing materials, such as metal hydrides, according to embodiments of the invention. The system  300  may include a source  302  of starting hydrogen-containing material that includes one or more impurities. The system  300  may also include a purifier unit  304  that contains the purifier material  306 . The system may further include components to activate the purifier material by heating to an activation temperature (e.g., about 100° C. or more, about 150° C. or more, etc.) in a dry atmosphere. These components may include a dry atmosphere source gas  308  and a temperature control unit  310  in thermal contact with the purifier material  306  and purifier unit  304 . 
     Embodiments of system  300  may include valves and gas flow conduits that allow the purifier unit to be switched between an activation stage and a purification stage. During the activation stage, the valves may be adjusted so only gas from the dry atmosphere source  308  flows into the purifier unit, which is being heated to the activation temperature by the temperature control unit  310 . After the dry atmosphere gas picks up moisture and other impurities from the purifier material, it may be pumped through a downstream outlet  312  on the purifier unit  304 , optionally with the assistance of pumping system  314 . Valves in fluid communication with the downstream outlet  312  may be adjusted to direct the dry atmosphere out of the system  300  instead of to the application component  316  or purified hydrogen material storage unit  318 . 
     After the purifier material has been activated (if an activation step is necessary) the system  300  may be configured for a purification stage. During this stage, valves may be adjusted to allow starting hydrogen-containing material from the hydrogen-containing material source  302  to flow into the purifier unit  304  and through the purifier material  306 . Optionally, the temperature control unit  310  may adjust the temperature of the purifier material  306  during the purification stage (e.g., adjusting the temperature of the purifier material to about 25° C. or less). Valves may be adjusted so that purified hydrogen-containing material exiting the downstream outlet  312  can optionally be sent to the application component  316  or the purified material storage unit  318 . The application component  316  may include, for example, a silicon epitaxial growth chamber for forming a silicon layer on a substrate using chemical vapor deposition. The purified material storage unit may include a storage tank or cylinder that may be reversibly decoupled from the system  300  so it can be transported to a storage location or point of use. 
     System  300  may be configured to allow multiple cycles of hydrogen-containing material purification and purifier material activation. In in-situ applications of system  300 , this can allow the purifier material to be recharged after a period of purification without having to physically replace the purifier material  306  from the purifier unit  304 . In additional embodiments, the purifier unit  304  may be configured to allow access to the purifier material  306  so that it can be supplemented or replaced by new purifier material. 
     EXPERIMENTAL 
     Experiments were conducted to measure composition changes in a specific hydrogen-containing material: trisilane (Si 3 H 8 ) after exposing samples of the starting gas to various purification materials. The starting trisilane compositions, already about 99.999% pure, were passed through purifier materials with different compositions and/or different activation temperatures. Moisture levels in the purified samples were measured using cavity ring down spectroscopy (CRDS) and compared to the moisture levels in the unpurified samples, as well as each other. Comparisons in the levels of additional impurities in the trisilane samples, including mono-, di-, and tetrasilanes, were also made using gas chromatography/mass spectroscopy (GC-MS). 
     Moisture Analyzer &amp; GC Gas Manifold Experimental Setup 
     A schematic of the experimental setup for measuring the impurity levels in trisilane samples is shown in  FIG. 4 . The schematic shows a gas manifold designed to take samples of trisilane from a lecture bottle  402 . When the trisilane samples are mixed with nitrogen (N 2 ) (for example when performing a GC-MS analysis of the sample) the nitrogen is supplied by nitrogen source  404 . Purified trisilane sample vapor may be generated by sending the sample through one or more purifier material  406   a - b . The manifold is also configured to have unpurified samples bypass the purifier materials  406   a - b  for comparative measurements of the impurities levels. 
     After a trisilane sample either passes through or bypasses the purifier material, it flows to analysis equipment for impurities measurements. The manifold valves may be configured to flow the sample directing into a cavity ring down spectrometer  408  to measure the moisture content in the sample. Alternatively, the manifold valves may be configured to flow the sample into a gas chromatogram-mass spectrometer (not shown) to measure the relative amounts of trisilane and other silanes (e.g., monosilane, disilane, iso-tetrasilane, n-tetrasilane, etc.). The sample may then return from the GC-MS for additional measurements in the CRDS, or may flow to the exhaust section of the manifold  410 . 
     In the exhaust section  410  of the manifold, the samples may be run through a series of cold traps  412  to capture condensable waste materials and then pass through a scrubber  414  before being vented out of the manifold. 
     Moisture and GC Analysis 
     To prepare for the trisilane moisture measurements, purified N 2  is used to purge and dry-down the manifold, and the manifold pressure is maintained at approximately 1000 torr. The pressure of the ring-down cavity was set for 250 torr and 30° C. to allow maximum N 2  flow through the cavity cell. The manifold and the CRDS usually dry down within one or two days. 
     During the measurements, the trisilane sample vapor travels through the manifold and either passes through the purifier material, or bypasses it. The trisilane sample may also pass through the GC or bypass this instrument and head directly to the moisture analyzer. After measurements are taken in the moisture analyzer, the trisilane sample vapor passes to the scrubber. When GC analyses are also performed, the trisilane sample vapor can pass through the GC sample loop before entering the moisture analyzer. The GC response is calibrated using a 10 ppm disilane in helium standard. 
     Description of the Purifier Materials Tested 
     The purifier materials tested included three materials: silicates with amorphous structure (Material A), and crystalline silicates (Materials B &amp; C). Some physical characteristics, capacity and efficiency data for the tested silica gels and crystalline silicates are given in Table 1: 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Characteristics and Efficiency Data for Selected Silicas: 
               
            
           
           
               
               
               
               
            
               
                   
                 Material A 
                 Material B 
                 Material C 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Activation Temp [° C.] 
                 150 
                 400 
                 400 
                 400 
               
            
           
           
               
               
               
               
            
               
                 Structure 
                 Amorphous 
                 Crystalline 
                 Crystalline 
               
               
                 Surface Area [m 2 /g] 
                 300 
                   
                 400 
               
               
                 Pore size [Å] 
                 100 
                 4 
                 6 
               
               
                 pH 
                 7.4 
                 11 
                 10.5 
               
               
                 SiO 2 /AI 2 O 3  (mol ratio) 
                 12000 
                 1-10 
                 800-1000 
               
            
           
           
               
               
               
               
               
            
               
                 H 2 O Capacity in N 2   
                 9.11 
                 2.79 
                   
                 5 
               
            
           
           
               
               
               
               
            
               
                 [L/L] 900 ppm 
                   
                   
                   
               
               
                 challenge 
                   
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 H 2 O efficiency in N 2   
                   
                 &lt;10 ppb 
                   
                 &lt;10 ppb 
               
               
                 (H 2 O Sensor) 
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     The Material B was already in an activated state from the manufacturer. The Material C was activated at 400° C. or 24 hours in 1 slpm N 2  prior to the experiments on a separate manifold at a different location. The activation treatment of the Material A includes thermal activation at two different temperatures: 150° C. and 400° C. for 24 hrs in 1 slpm N 2 . Moisture capacity and efficiency of the silica materials were measured using moisture sensors on a separate manifold. 
     Experimental Results 
     As expected, not all silicon containing materials were ideal for trisilane purification. Of the purifier materials tested, the Material A activated at 400° C. for 24 hours in 1 slpm of N 2  appeared most effective at reducing moisture and tetrasilane levels in a trisilane vapor stream. For Material A, the thermal activation above 150° C. enhanced moisture removal and reduced decomposition of the trisilane. While not wishing to be bound to a particular theory of how the thermal activation process works, it is believed heating these materials above 150° C. converts reactive silanol (Si—OH) groups on the surface of the materials into less reactive Si—O—Si groups. 
       FIG. 5  shows a plot of the decreasing surface density of Si—OH groups (in atoms/nm 2 ) as a function of the Material A activation temperature. The amount of Si—OH was observed to decrease significantly when the activation temperature is set from about 200° C. to about 400° C. It is believed that the Si—OH groups readily react with the trisilane to form Si x H y  decomposition products and hydrogen gas. Thus, by decreasing the density of the Si—OH groups on the purifier surface through thermal activation reduces the decomposition of the trisilane as well as increases the sites available for capturing moisture in the trisilane vapor. 
     Another reactive species present in many trisilane purifier materials is aluminum (Al), usually occurring in oxidized form as alumina (Al 2 O 3 ). Aluminum is very reactive with silane compounds and can quickly decompose higher silanes like trisilane. Alumina (Al 2 O 3 ) has been shown to decompose silanes and alkyl silanes. Thus, the trisilane purification results were also compared with the relative amounts of silica (SiO 2 ) to alumina (Al 2 O 3 ) (i.e., the SiO 2 :Al 2 O 3  ratio) in the purification material. Material A had a high SiO 2 :Al 2 O 3  ratio of about 12000, while Material B &amp; C had substantially lower SiO 2 :Al 2 O 3  ratios: The Material B had a ratio of about 10, and Material C had a ratio of about 800-1000. 
     Results of the Moisture Challenge Tests 
     As noted above, the capacities of the examined purifier materials were measured using a 900 ppm H 2 O challenge in 1 slpm N 2  using a moisture generator. The Material A activated at 400° C. had the lowest capacity at 2.79 L H 2 O per liter of purifier material (L/L). 
     The same Material A activated at 150° C. had a better capacity at 9.11 L/L and the Material C had a capacity of about 5.0 L/L. The capacity of Material B was not determined. 
     The Material A, B, &amp; C had a moisture efficiency of 10 ppb or lower as summarized in Table 1 supra. 
     Experimental Results 
     1. Material A 
     The water concentration in the neat/pure trisilane vapor that bypassed the purifier material was about 6500 ppb. When the trisilane sample was instead flowed through the Material A activated at 400° C., the water vapor impurity concentration dropped from 6500 ppb to about 35 ppb in the purified trisilane stream, an over 100-fold reduction.  FIG. 6  shows the time plot spectra of water vapor concentration over a cycle of unpurified trisilane and purified trisilane passing through the activated Material A. 
     2. Material B 
     As noted above, the water concentration in the neat/pure trisilane was approximately 6500 ppb when bypassing the purifier material. When the trisilane was then flowed through the Material B purifier, the water concentration dropped to about 300 ppb, over an order of magnitude decrease in the water vapor concentration. 
     3. Material C 
     Using a source of trisilane from Matheson Tri-Gas, the water vapor concentration in the unpurified neat source sample was approximately 500 ppb, as measured in a sample that by passed the purifier material. When the trisilane then flowed through the Material C, the water vapor concentration actually increased to approximately 2000 ppb. 
     GC-MS Experimental Results 
     Neat trisilane that bypassed the purifier material was analyzed by the GC and the resulting chromatogram shown in  FIG. 7  indicated that the largest peak (peak 5) was the trisilane. Other impurities were identified by retention time and molecular weight. Peaks 1, 3, 6, and 7 in the chromatogram correspond to monosilane, disilane, iso-tetrasilane, and n-tetrasilane, respectively. Peaks 2 and 4 are siloxanes. Based on calibrations using a 10 ppm disilane in He standard, the trisilane contained about 27.5 ppm of monosilane and 191 ppm of disilane. The concentrations of the siloxanes, iso-tetrasilane, and n-tetrasilane were estimated to be 2.5, 2.7, and 5.1 ppm using the same disilane calibration standard. All trisilane samples received were analyzed to confirm the level of impurities. Another trisilane source from Matheson Tri-Gas showed higher impurity levels, which were believed to be the result of natural decomposition over time. Table 2 summarizes the impurity levels measured by GC in the trisilane source used. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Impurity Levels in Trisilane Calculated Based on Response  
               
               
                 Factor of 10 ppm Disilane Standard 
               
            
           
           
               
               
               
               
            
               
                   
                 Compound 
                 % Assay 
                 Conc (ppm) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 SiH 4   
                 0.098 
                 27.5 
               
               
                   
                 Si 2 H 6   
                 0.684 
                 191 
               
               
                   
                 Si 2 H 6 O 
                 0.009 
                 2.5 
               
               
                   
                 Si 3 H 8   
                 99.182 
                 — 
               
               
                   
                 i-Si 4 H 10   
                 0.01 
                 2.7 
               
               
                   
                 n-Si 4 H 10   
                 0.018 
                 5.1 
               
               
                   
                   
               
            
           
         
       
     
     1. Material A 
     GC-MS impurity analyses were done on trisilane samples that passed through Material A activated at 400° C. for 24 hours in 1 slpm N 2 . The GC-MS analysis of the trisilane samples passed through the activated Material A showed that substantially all the iso-tetrasilane and n-tetrasilane was removed. The analysis also indicated a slight increase in the concentration of monosilane from about 12 ppm to about 17 ppm, and an increase in disilane from about 251 ppm to about 666 ppm. These increased in mono- and disilane were believed to be caused in part by the decomposition of higher silanes (including trisilane) that may have come into contact with Si—OH groups on the silica gel.  FIG. 8  is a graph showing the concentration changes in various impurities for trisilane samples that bypassed or passed through a Material A purifier thermally activated at 400° C. 
     A comparative test was also performed to measure the effect of activation temperature on the impurity levels for Material A. Material A activated at 150° C. for 24 hours in 1 slpm of N 2  generated larger concentrations of monosilane (approximately 4500 ppm) and disilane (approximately 12,000 ppm) than the Material A activated at higher temperature. However, the low temperature activated Material A also removed the tetrasilanes below the level of detection. The comparison of the 150° C. and 400° C. temperature activated Material A support the theory that more of the Si—OH groups are converted to Si—O—Si upon heating at higher temperature. The lower density of Si—OH groups at 400° C. results in less of the trisilane being converted to mono- and disilane. 
     2. Material B 
     Significant amounts of impurities were generated form trisilane when it was exposed to the Material B. This material decomposed the trisilane to significant amounts of monosilane (approximately 100 ppm), disilane (approximately 500 ppm), as well as converting the trisilane to iso-tetrasilane (approximately 200 ppm) and n-tetrasilane (approximately 700 ppm), and higher molecular weight impurities (approximately 400 ppm). Material B had the highest concentrations of aluminum of all the purifier materials tested. Thus, the extent of trisilane decomposition and conversion to higher silanes supports the idea that Al plays a role in forming reactive silane species (possibly SiH 3   −  and/or SiH 3 .). These reactive silane ions and radicals significantly accelerate the decomposition and conversion of trisilane to other compounds, including higher silanes like iso-tetrasilane, n-tetrasilane, and pentasilane (Si 5 H 12 ). The ratio of iso-tetrasilane to n-tetrasilane observed in the chromatogram is 1:2, indicating this reaction is not selective. 
     3. Material C 
     The Material C purifier removed about 95% of the tetrasilanes, but generated significant amounts of monosilane (approximately 150 ppm) and disilane (approximately 1400 ppm), as well as some higher molecular weight impurities (approximately 400 ppm). 
     Purifier Conditioning Tests 
     During the conditioning test, it took approximately 25 minutes for neat trisilane to condition a Material A purifier activated at 400° C. The temperature of the silica gel purifier remained at room temperature for the duration of the conditioning test. By comparison, another conditioning test was performed by exposing neat trisilane to an Material C purifier for more than half an hour. During this time, the Material C purifier continuously chemically absorbed trisilane and the CRDS indicated that no trisilane vapor flowed through the purifier. Meanwhile, the temperature of the purifier increased from room temperature (approximately 25° C.) to about 40° C. The temperature increase indicates that the Material C is absorbing trisilane, and may also explain why more water vapor appears to be emitted from this purifier material than is being removed. 
     Experimental Summary 
     Table 3 below summarizes the performance results for three purifier materials analyze in these experiments. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Summary of Purification Performance for Tested Materials 
               
            
           
           
               
               
               
               
            
               
                   
                 Material A 
                 Material B 
                 Material C 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Temperature  
                 150° C. 
                 400° C. 
                 400° C. 
                 400° C. 
               
               
                 of Activation 
                   
                   
                   
                   
               
               
                 Al Concentration 
                 ~70 ppm 
                 ~70 ppm 
                 10% 
                 1000 ppm 
               
               
                 Number of  
                 6.2 
                 2.7 
                 N/A 
                 N/A 
               
               
                 Si—OH 
                   
                   
                   
                   
               
               
                 (atom/nm 2 ) 
                   
                   
                   
                   
               
               
                 Capacity Data 
                 Good 
                 Good 
                 N/A 
                 Good 
               
               
                 in N 2   
                   
                   
                   
                   
               
               
                 Efficiency  
                 Poor  
                 Good 
                 Poor  
                 Bad, Emits 
               
               
                 Data in 
                 Efficiency 
                 Efficiency 
                 Efficiency 
                 H 2 O, 
               
               
                 Trisilane 
                 600 ppb  
                 35 ppb 
                 300 ppb 
                 2000 ppb 
               
               
                   
                 through 
                 through 
                 through 
                 through  
               
               
                   
                 purifier, 
                 puffier, 
                 purifier, 
                 purifier, 
               
               
                   
                 6500 ppb 
                 6500 ppb 
                 6500 ppb 
                 800 ppb 
               
               
                   
                 through 
                 through 
                 through 
                 through 
               
               
                   
                 by-pass. 
                 by-pass. 
                 by-pass. 
                 by-pass. 
               
               
                 Decomposition 
                 Generate  
                 Generate 5 
                 Generate  
                 Generate  
               
               
                   
                 ~4500  
                 ppm silane 
                 ~100 
                 ~150 
               
               
                   
                 ppm silane  
                 &amp; ~400 
                 ppm  
                 ppm silane, 
               
               
                   
                 &amp; ~12,000  
                 ppm 
                 silane, 
                 ~1400 ppm 
               
               
                   
                 ppm  
                 disilane. 
                 ~500 ppm 
                 disilane, &amp; 
               
               
                   
                 disilane. 
                 Remove 
                 disilane,  
                 ~400 ppm 
               
               
                   
                 Remove  
                 100% of 
                 ~900 
                 high MW  
               
               
                   
                 80-90% 
                 tetrasilanes. 
                 ppm 
                 impurities. 
               
               
                   
                 tetrasilanes,  
                   
                 tetrasilanes,  
                 Remove  
               
               
                   
                 &amp; 35% 
                   
                 &amp; ~400 
                 ~90% of  
               
               
                   
                 of high 
                   
                 ppm high 
                 tetrasilanes. 
               
               
                   
                 MW  
                   
                 MW  
                   
               
               
                   
                 impurities. 
                   
                 impurities. 
                   
               
               
                 Effect of 
                 At 150° C., 
                 At 400° C., 
                 N/A 
                 N/A 
               
               
                 Activation  
                 silane &amp; 
                 silane &amp; 
                   
                   
               
               
                 Temp on  
                 disilane 
                 disilane 
                   
                   
               
               
                 decomposition 
                 impurities is 
                 impurity is 
                   
                   
               
               
                   
                 many times 
                 very low in 
                   
                   
               
               
                   
                 compared to 
                 conc. at 
                   
                   
               
               
                   
                 at 400° C. 
                 150° C. 
                   
                   
               
               
                 Effect  
                 N/A 
                 At ~70 ppm 
                 At 10%  
                 At 1000  
               
               
                 of Al 
                   
                 Al conc., 
                 Al conc.,  
                 ppm Al  
               
               
                 concentration 
                   
                 tetrasilanes 
                 ~1300 
                 conc., ~400 
               
               
                 on  
                   
                 &amp; higher 
                 ppm of 
                 ppm of 
               
               
                 decomposition 
                   
                 MW 
                 tetrasilanes  
                 tetrasilanes  
               
               
                   
                   
                 impurities 
                 &amp; higher  
                 &amp; higher  
               
               
                   
                   
                 were not 
                 MW 
                 MW 
               
               
                   
                   
                 observed. 
                 impurities  
                 impurities  
               
               
                   
                   
                   
                 were 
                 were 
               
               
                   
                   
                   
                 observed. 
                 observed. 
               
               
                 Chemisorption/ 
                 No- 
                 No- 
                 N/A 
                 Yes,  
               
               
                 Temperature of 
                 material 
                 material 
                   
                 absorbs 
               
               
                 the purifier on 
                 conditioned 
                 conditioned 
                   
                 trisilane.  
               
               
                 exposure to 
                 rapidly 
                 rapidly 
                   
                 Temp 
               
               
                 Trisilane 
                   
                   
                   
                 increased to 
               
               
                   
                   
                   
                   
                 ~40° C. 
               
               
                   
               
            
           
         
       
     
     The Material A purifier activated at 400° C. showed the best performance of all the trisilane purifier materials tested, but many of the other purifier materials are also suitable for reducing moisture and other impurities in trisilane to levels that are acceptable for many modern electronic and semiconductor fabrication applications. For example, silica gels activated at lower temperatures (e.g., 150° C.) and the Material B purifier may be used in applications were moisture reduction is more important than increased levels of lower silanes (e.g., monosilane and disilane). 
     All the purifier materials tested decomposed trisilane to some degree. Those that were more concentrated in aluminum (as reflected by a lower SiO 2 :Al 2 O 3  ratio) tended to decompose the trisilane to a greater extent. Those with higher SiO 2 :Al 2 O 3  ratios were more inert to trisilane. As noted above, the selection of a trisilane purifier material may depend on the ultimate end use of the trisilane, and the extent to which a tradeoff can be made between lower levels of moisture and increased levels of lower silanes. 
     Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Additionally, a number of well-known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. Accordingly, the above description should not be taken as limiting the scope of the invention. 
     Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included. 
     As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a process” includes a plurality of such processes and reference to “the purifier” includes reference to one or more purifiers and equivalents thereof known to those skilled in the art, and so forth. 
     Also, the words “comprise,” “comprising,” “include,” “including,” and “includes” when used in this specification and in the following claims are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, acts, or groups.