Abstract:
A method for purifying a precursor material for use in a chemical vapor deposition system comprising providing a precursor material suitable for chemical vapor deposition, and distilling the precursor material to provide, as a distillate, a purified precursor material from which at least one non-volatile component, at least one metal impurity, or a combination thereof has been removed. The method may be used to purify siloxane and silane precursor materials and reduce clogging of chemical vapor deposition components.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of the filing date of U.S. Provisional Application Ser. No. 60/822,401, filed Aug. 15, 2006, the entire disclosure of which is incorporated herein by reference. 
     
     TECHNICAL FIELD 
       [0002]    The present invention relates broadly to a method for purifying precursor materials for use in chemical vapor deposition processes and a system for purifying precursor materials. 
       BACKGROUND 
       [0003]    High purity fluid processing applications may be employed to deposit high purity films, such as metal based films, on a substrate. Examples of such fluid processing applications include spray coating, spin coating, sol-gel deposition, chemical vapor deposition (CVD), and the like. Chemical vapor deposition is an increasingly utilized delivery process for forming solid materials, such as coatings or powders, by way of reactants in a vapor phase. Typically, a precursor material is vaporized by heating to an appropriate temperature and bubbling a flow of carrier gas through the liquid to transport the vapor into a CVD chamber. The vapor is transported to a substrate and contacted with the substrate under conditions sufficient to deposit a thin film onto the substrate. 
         [0004]    Chemical vapor deposition is widely used in the semiconductor manufacturing industry to provide a metal coating on a semiconductor device. For a semiconductor device to perform properly, it is best for the coating to have a very high purity. Consequently, the precursor material to be used in the chemical vapor deposition process should have a very high purity (on a metal contaminants basis) such as, for example, at least 99.5% and even 99.99% or higher. Precursors, which are typically obtained in bulk from a supplier, are often expensive materials, and the cost increases for higher purity materials. 
         [0005]    In addition to the high cost associated with high purity precursors, certain classes of precursors may present problems in the delivery of the precursor during the chemical vapor deposition process. Silanes and siloxanes have been used for precursor materials for performing CVD on a substrate. But a problem with these materials has been formation of a residue that clogs delivery tubes and other components in the CVD system. The clogging causes unreliable or non-reproducible delivery of precursor material to the CVD system, which may cause a high field failure rate of mechanisms in liquid delivery systems. This problem has perplexed those in the industry, as the silane and siloxane precursor materials are typically high purity materials often having a purity of at least about 99.99% 
       SUMMARY 
       [0006]    The present invention provides a method and system for purifying precursor materials prior to introduction into a high purity fluid processing application such as, for example, chemical vapor deposition. In general, the method includes providing a precursor material having one or more impurities and distilling the precursor material to produce, as a distillate, a purified precursor material. 
         [0007]    The method includes providing a precursor material suitable for chemical vapor deposition, and distilling the precursor material to provide, as a distillate, a purified precursor material from which at least one non-volatile component, at least one metal impurity, or a combination thereof has been removed. The present invention provides a method that reduces, if not eliminates, clogging of CVD components by distilling a precursor material to remove impurities that have been found to be the source of the clogging. This is surprising in that prior processes used high purity precursor material including material having a purity of at least 99.99%. 
         [0008]    The method is carried out by a distillation process and does not require the distillation to be an azeotropic distillation. The distillation may be performed as a vacuum distillation, by applying a vacuum to the distillation system and carrying out the distillation at a pressure below atmospheric pressure. The distillation may be carried out at a pressure below about 150 torr. 
         [0009]    The precursor material may be any material suitable for depositing a desired film on a substrate via chemical vapor deposition. The method is suitable for use with siloxane and silane precursor materials such as, for example, tetramethylcyclotetrasiloxane, tetraethoxysilane, and the like. 
         [0010]    The method provides for the removal of non-volatile impurities from precursor materials that may clog the components of a chemical vapor deposition apparatus. The method also provides for the removal of metal impurities that may be considered as contaminants if deposited on the substrate during a chemical vapor deposition. The method may allow less expensive, lower purity precursor materials to be purchased from suppliers and subsequently purified to provide a precursor material of a desired purity. 
         [0011]    The purification may be performed at the precursor supplier&#39;s site or at the same location at which the chemical vapor deposition process is carried out. The distillation may be performed using a self-contained distillation apparatus. The self-contained distillation system may be used as a filling station to collect purified precursor material for use at a later time. The self-contained distillation system may also be connected to a chemical vapor deposition apparatus to supply the purified precursor material directly from the distillation apparatus to a chemical vapor deposition apparatus. The system may be configured to provide a substantially continuous flow of purified precursor material to a chemical vapor deposition apparatus. 
         [0012]    The present invention also provides a method for depositing a coating on a substrate by chemical vapor deposition by delivering a purified precursor material obtained by the purification process in accordance with the present invention to a chemical vapor deposition apparatus, vaporizing the purified precursor, and contacting the purified precursor vapor with a substrate to deposit a desired coating on the substrate. 
         [0013]    The present invention also provides a chemical vapor deposition system comprising a distillation system for purifying a precursor material; and a chemical vapor deposition apparatus; wherein the distillation system comprises a distillation vessel; a fluid inlet in fluid communication with the distillation vessel for delivering a precursor material to the distillation vessel; a condenser; at least one receiver for receiving a purified precursor material obtained by distilling a precursor material; a gas inlet in fluid communication with the at least one receiver for introducing a gas into the at least one receiver and pressurizing the at least one receiver to facilitate the flow of the purified precursor material to the high purity fluid processing system; and an outlet in fluid communication with the chemical vapor deposition apparatus. 
         [0014]    The foregoing and other features of the invention are hereinafter described in detail in conjunction with the accompanying drawings which set forth exemplary embodiments illustrating a few of the various ways in which the principles of the invention may be employed. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    In the annexed drawings: 
           [0016]      FIG. 1  is a schematic representation of an exemplary chemical vapor deposition system employing an distillation system for purifying a precursor material in accordance with the present invention; and 
           [0017]      FIG. 2  is a perspective view of an exemplary distillation system for purifying a precursor material. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The present invention provides a method and system for purifying precursor materials prior introduction into a high purity fluid processing application such as, for example, chemical vapor deposition. In general, the method includes providing a precursor material having one or more impurities and distilling the precursor material to produce, as a distillate, a purified precursor material. 
         [0019]    An exemplary method and system for purifying a precursor material may be understood with reference to  FIGS. 1 and 2 . With reference to  FIG. 1 , an exemplary distillation system  100  includes a distillation vessel  108 , a distillation column  116 , a condenser  120 , a first receiver  128 , and a second receiver  134 . Generally, a precursor material is supplied from a precursor feedstock  102  through a line or inlet  104  to the distillation vessel  108 . The precursor material is heated, such as by heating the vessel  108  using a heater jacket  110 , to a suitable temperature to cause the precursor material to vaporize. The vapor climbs up the column  116  and condenses at the junction between the column  116  and the condenser  120 . The condenser tube is cooled, such as by cooling fins  122 , and the precursor vapor condenses back to a liquid and passes through a line  124  and into a receiver such as one of receivers  128  or  134  via lines  130  or  136 , respectively. The liquid received in the receivers  128  and/or  134  is a purified precursor material. 
         [0020]    The purified precursor material may then be transported to a high purity fluid processing application such as, for example, a chemical vapor deposition system. As shown in  FIG. 1 , the distillation system  100  may be coupled to a chemical vapor deposition apparatus  148 . The distillation system may include lines  140  and  142  for delivering purified precursor material from the receivers  128  and  134 , respectively, to the chemical vapor deposition apparatus  148  via an outlet or line  146  in fluid communication with the chemical vapor deposition apparatus  148 . 
         [0021]    Distilling the precursor liquid may be accomplished by any suitable distillation technique. The distillation step may include simply heating the precursor material to a desired temperature and collecting, as a distillate, a purified precursor material. The distillation step may also include heating the precursor material to a first temperature below the boiling point of the precursor material (at a selected pressure), and then subsequently heating at a second temperature at about the boiling point of the precursor material (at a selected pressure). The distillate produced at the first, lower temperature may be collected along with the distillate from the distillation at the second temperature as purified precursor material. Alternatively, the distillate at the lower temperature, which may include lower boiling components, may be discarded (such as by evacuation or sent to the waste receptacle). It is not necessary to form an azeotropic mixture in the precursor material and then distill the precursor via azeotropic distillation to obtain a purified precursor material in accordance with the present invention. 
         [0022]    The distillation may be carried out at any suitable temperature depending on the boiling point of the precursor material being used. The pressure inside the distillation vessel may be adjusted to lower the boiling point of the precursor material to prevent degradation of the precursor. The distillation vessel may be evacuated to provide a pressure below atmospheric pressure (760 torr) such as, for example, below about 200 torr, below about 150 torr, below about 100 torr, below about 50 torr, or even lower. 
         [0023]    Additionally, the distillation vessel may be purged to remove oxygen from the system and apply a blanket of inert gas over the precursor liquid to prevent oxidation of the precursor. Suitable inert gases include, but are not limited to, nitrogen, helium, and the like. 
         [0024]    Referring back to  FIG. 1 , a distillation system such as system  100  may be provided to supply a chemical vapor deposition apparatus with a substantially continuous flow of purified precursor material. By opening a valve  126  to lines  130  and  134 , receivers  128  and  134  may be in fluid communication with the line  124 , the condenser  120 , the column  116 , and the distillation vessel  108 . The distillation vessel  108  and the receivers  128 ,  134  may be brought to a desired pressure by opening the valve  126  to lines  130  and  134 , opening the valves  160  and  162  to the lines  164  and  166 , respectively, and applying a vacuum to the system using a vacuum pump  156 . Upon reaching the desired pressure, the vacuum  156  is shut off and the valves  160  and  162  are closed. 
         [0025]    Precursor material is then distilled, and purified precursor vapor is condensed in the condenser  120  and passes through the line  124  to receivers  128  and  134 . The valve  126  may be used to control the flow of purified precursor material to receivers  128  and  134  through lines  130  and  136 , respectively. Receivers  128  and  134  may include a level sensor (e.g., level sensors  132  and  138 , respectively, and upon reaching a desired level in a receiver, the valve  126  may be closed to stop the flow of purified precursor liquid to one of the receivers. Generally, the receivers are filled one at a time. 
         [0026]    The system may contain a gas inlet or delivery line  170  in fluid communication with a gas source  168  and receivers  128  and  134  through delivery lines  176  and  178 , respectively. The system may also include valves  172  and  174  to control the flow of gas into the receivers  128  and  134 , respectively. After receivers the  128  and  134  are filled to a desired level with purified precursor material, the receivers may be pressurized by applying a gas blanket from the gas source  168  over the liquid in the receiver. The gas and pressure in the receiver may facilitate the flow of purified precursor from the receivers to the chemical vapor deposition apparatus. The gas applied to the receiver may be any suitable gas including, for example, helium, argon, hydrogen, and the like. A CVD apparatus typically employs a push pressure gas, such as helium, to facilitate the flow of precursor material whereby the carrier gas is mixed with the liquid precursor prior to entering a vaporizer or whereby the carrier gas is mixed with precursor vapor after the liquid precursor has been vaporized in a vaporizer. The apparatus shown in  FIG. 2  may be connected to a carrier gas source on site at a CVD shop or plant and the push pressure gas used for the CVD apparatus may be used to pressurize the receivers to provide the push pressure to flow the precursor material to the CVD apparatus. The pressure in the receiver may be selected as desired for a particular purpose or intended use, such as to provide a desired flow rate from the receiver to a CVD apparatus. For example, the pressure may be applied with helium at about 20 to about 70 psig. 
         [0027]    Purified precursor may be delivered to the chemical vapor deposition apparatus  148  by opening a delivery valve  144  to allow the purified precursor to flow through lines  140  or  142  through an outlet or delivery line  146  to the chemical vapor deposition apparatus. The purified precursor material is pushed from the pressurized receivers  128  or  134 . To provide a continuous flow of purified precursor to the chemical vapor deposition apparatus, the purified precursor material is generally delivered from one receiver at a time. When the purified precursor is depleted to a selected volume, the system is transitioned to deliver precursor from the other receiver. For example, after supplying purified precursor material from receiver  128  and reaching a selected low level of purified precursor, the valve  144  is opened to line  142  to allow purified precursor from receiver  134  to flow into the outlet/line  146 . For a selected period of time, the valve  144  also may remain open to the line  140  such that purified precursor is delivered from both receiver  128  and receiver  134 . After the transition period, the delivery valve  144  is closed to line  140  and purified precursor is delivered solely from receiver  134 . 
         [0028]    A substantially continuous flow of purified precursor material may be provided by refilling the receiver having a depleted volume of purified precursor material while another full receiver delivers material to a CVD apparatus. For example, while purified material is being delivered from receiver  134 , receiver  128  may be conditioned (such as by applying a vacuum to the receiver to provide a desired low pressure) and refilled by distilling precursor material and directing the flow into receiver  128 . If necessary, the distillation vessel may be conditioned (e.g., by emptying any waste and evacuating the distillation vessel to provide a desired pressure level) and refilled with bulk precursor material prior to refilling an empty receiver. When the receiver  134  reaches a desired low level of precursor material, the system may be transitioned to deliver purified material from refilled receiver  128 , and receiver  134  may then be refilled. To provide a substantially continuous flow, it may be desirable that the flow rate of the distillation process be about equal to the flow rate of purified precursor material to the high purity fluid processing system. 
         [0029]    After distilling the precursor material, the distillation vessel may contain some undistilled material and waste residue. With reference to  FIG. 1 , the undistilled material and waste residue may be removed from the distillation vessel  108  into a waste receptacle  150 . The waste may enter the waste receptacle  150  through a line  152  by opening a valve  154 . The waste may be removed by any suitable method including by gravity, by vacuum, or by pressure. The waste could be forced through line  152  by introducing a push pressure into the distillation vessel through one of the receivers. For example, valve  126  could be opened to line  130  such that receiver  128  is in fluid communication with line  124 , condenser  120 , column  116 , and the distillation vessel  108 . Valve  172  may then be opened to line  176  and a push pressure may be introduced into the distillation vessel by introducing a gas from gas source  168  into receiver  128 , which will then flow through lines  130 , line  124 , condenser  120 , column  116  and into distillation vessel  108  to provide a pressure or flow of gas to push the waste into the waste receptacle. It will be appreciated that the distillation apparatus need not be emptied prior to refilling. Rather, precursor material may be introduced into the distillation vessel upon reaching a selected low level of material in the distillation vessel. 
         [0030]    The distillation system may be automated and include any sensors (e.g., level sensors  112 ,  132 , and  138  for monitoring the level of liquid in the distillation vessel and the receivers, spill sensors, leak sensors, or the like), temperature probes (e.g., probes  114  and  118 ), pneumatic banks (e.g., pneumatic control bank  180 ), and management of clean dry air for pneumatically actuated valves, interlocks, tubing, hardware (e.g., touch screen  182  and programmable logic controller  184 ), software, and/or circuitry necessary to operate the system and provide high purity liquid to meet the desired requirements of the high purity fluid processing system (e.g., chemical vapor deposition tool). The distillation system could be programmed as desired to control the operation of the system such as, for example, automatically supplying precursor material to the distillation vessel. As another example, the distillation of a precursor material could be stopped upon reaching a selected low level of material in the distillation vessel and not restarted until a signal is received that a receiver is at a selected low volume of purified precursor material and needs to be refilled. 
         [0031]    As shown in  FIGS. 1 and 2 , the distillation system includes two receivers. The distillation system, however, may include one, two, three, or more receivers for collecting purified precursor material, and may include two or more receivers. 
         [0032]    As shown in  FIG. 1 , the distillation system  100  is connected to chemical vapor deposition apparatus. The chemical vapor deposition apparatus may include any items necessary to deposit a film on a substrate such as, for example, a vaporizer, a vapor deposition reactor, and a delivery line to transport the purified precursor vapor to the deposition reactor. The purified precursor may be flowed to a CVD apparatus, combined with a carrier gas, flowed to a vaporizer, vaporized, and then flowed to a reactor for deposition on to a substrate. Alternatively, the purified precursor may be flowed to a CVD apparatus, vaporized, combined with a carrier gas, and the vapor/carrier gas or mixture may be flowed to a reactor for deposition on to a substrate. The purified precursor vapor is contacted with a substrate under conditions sufficient to deposit a thin film of a desired composition on the substrate. The vapor may be transported to the deposition reactor by the carrier gas used to push the liquid precursor to the deposition apparatus or an additional flow of gas may be provided to the chemical deposition system to transport the purified precursor vapor to the reactor. The chemical deposition apparatus may include more than one reactor, and the purified material from one distillation system may be sufficient to supply multiple reactors. The chemical vapor deposition apparatus may include any other instrumentation or devices as necessary to carry out the vapor deposition process. 
         [0033]    It will be appreciated that the distillation system does not have to be connected to a chemical vapor deposition apparatus. Rather, the distillation system may serve as a “filling station” to collect purified precursor material in a receiver. A receiver could be disconnected from the distillation system and moved to a desired location where it may be hooked up for local delivery of purified precursor material. 
         [0034]    The precursor material may be any material as desired for a particular purpose or intended use. A particularly suitable class of precursor materials includes silicone based precursors. Suitable silicone based precursors include, but are not limited to, alkyl silanes alkoxy silanes, siloxanes silsesquioxanes, and the like. An example of a suitable alkoxy silane is tetraethoxysilane (TEOS). Suitable silsesquioxanes include, for example, polyhedral oligomeric silsesquioxanes. Suitable siloxanes include, but are not limited to, cyclosiloxanes such as, for example, octamethylcyclotetrasiloxane, hexamethylcyclotetrasiloxane, tetramethylcyclotetrasiloxane, and the like. The method in accordance with the present invention may remove non-volatile impurities from such materials, which may prevent clogging of components in a CVD apparatus. 
         [0035]    Prior to purification the precursor material by a method in accordance with the present invention, the bulk precursor material may have a purity, on a metals basis, of about 98% or higher, about 99% or higher, about 99.5% or higher, or even about 99.99% or higher. The method in accordance with the present invention allows a processor to purchase bulk precursor material of a relatively low purity (e.g., 98%) and purify the material as needed to supply to the processing system. Higher purity materials are generally more expensive, thus by purifying “on site” the cost of starting materials may be reduced. 
       EXAMPLE 
       [0036]    TEOS having a purity of 98% is purified via vacuum distillation as follows: TEOS is charged to a flask (distillation vessel) equipped with a condenser, which is connected to a receiver. The flask is evacuated and a nitrogen blanket is applied over the TEOS in the flask to provide a reduced pressure of about 0 to about 50 torr. The flask is heated to distill the TEOS and the boiling point of the TEOS is 45-47° C. After distillation, a residue is obtained in the distillation vessel and the distillate collected in the receiver is a purified TEOS. 
         [0037]    Samples of TEOS are flowed through a D2000i series liquid mass flow controller available from Porter Instruments (Hatfield, Pa.). Pre-distilled, 98% pure TEOS is utilized as a control and is flowed through the controller. After flowing the control TEOS through the controller, the controller contains some crystals or oily residue, which could clog the controller. After the purified (distilled) TEOS precursor material is flowed through the controller, the controller does not contain any significant amount of crystal material or oil residue. 
         [0038]    It is anticipated that certain changes may be made in the present invention without departing from the precepts involved herein. It is intended that all matter contained in the foregoing description shall be interpreted as illustrative and not in a limiting sense.