Abstract:
The present invention relates generally to the field of semiconductor device manufacturing, and more specifically to an apparatus and method for in-situ cleaning of a throttle valve in a chemical vapor deposition (CVD) system. In the exhaust flow control apparatus of the CVD system, which comprises a chamber isolation valve, throttle valve and vacuum pump, means are provided for introducing cleaning gases downstream of the chamber isolation valve and upstream of the throttle valve. Such means may include a cleaning isolation valve connected to a cleaning gas source. Means for generating a reactive plasma of the cleaning gases, just before the throttle valve, may also be provided. During cleaning of the throttle valve, the CVD chamber is isolated, by closing the chamber isolation valve, and cleaning gases are flowed into the throttle valve, by opening the cleaning isolation valve.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a division of U.S. patent application Ser. No. 09/876,587 filed Jun. 7, 2001. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to the field of chemical vapor deposition (CVD) in semiconductor device manufacturing, and more specifically to a method for in-situ cleaning of a throttle valve in a CVD system.  
       BACKGROUND OF THE INVENTION  
       [0003]     Chemical vapor deposition (CVD) processes are used widely in the manufacture of semiconductor devices. Generally, CVD involves exposing a semiconductor wafer to a reactive gas under carefully controlled conditions including elevated temperatures, sub-ambient pressures and uniform reactant gas flow rate, resulting in the deposition of a thin, uniform layer or film on the surface of the substrate. The undesired gaseous byproducts of the reaction are then pumped out of the deposition chamber. The CVD reaction may be driven thermally or by a reactant plasma, or by a combination of heat and plasma. A typical CVD system is a single-wafer system utilizing a high-throughput CVD chamber.  
         [0004]     A typical CVD process begins with heating of the CVD chamber to a temperature between about 250° C. and about 1,000° C. A semiconductor substrate is placed in the chamber on a receptor, typically known as a susceptor, which is generally made of ceramic or anodized aluminum. Next, reactant gases are introduced into the chamber, while regulating the chamber pressure. The chamber pressure may be controlled to as low as 1 torr up to as high as atmospheric pressure. The gases react in the chamber to form a deposition layer on the surface of the wafer.  
         [0005]     Chamber pressure is precisely controlled by an inlet flow regulating device which regulates the flow rates of the gases into the chamber, and by an exhaust flow control apparatus attached to the exhaust gas port of the chamber. The exhaust flow control apparatus typically consists of an isolation valve, a throttle valve and a vacuum pump. The isolation valve is typically connected directly to the exhaust gas port of the reaction chamber, and the throttle valve is typically installed downstream from the isolation valve at a distance of approximately 6-10 inches away from the reaction chamber exhaust port. The vacuum pump is installed downstream from both the isolation valve and the throttle valve. During a typical deposition process, the isolation valve remains open while the throttle valve cycles between the open and closed positions to regulate the gas pressure in the chamber. The position of the throttle valve is controlled by a servo-motor which is in turn controlled by a closed-loop control system based on feed-back signals from a pressure manometer mounted in the reaction chamber.  
         [0006]     In a typical deposition process, reactant gases enter the reaction chamber and produce films of various materials on the surface of a substrate for various purposes, such as for dielectric layers, insulation layers, etc. The various materials deposited include epitaxial silicon, polysilicon, silicon nitride, silicon oxide, and refractory metals such as titanium, tungsten and their silicides. Most of the material produced from the reactant gases is deposited on the wafer surface. However, some material also is inevitably deposited on other surfaces inside the chamber, and some material also may be deposited on the throttle valve. Deposition of unwanted film on the throttle valve is more likely during deposition of certain materials, such as silicon oxide, which require a relatively high chamber pressure. As unwanted material is deposited on the throttle valve, the precise operation of the throttle valve is diminished, thereby compromising the precise control of the reactant gas pressure inside the reaction chamber.  
         [0007]     In a typical CVD system, after each deposition process wherein a film is deposited onto a semiconductor substrate and the substrate is removed from the chamber, a cleaning gas or mixture of cleaning gases is purged through the reaction chamber in order to clean unwanted deposits from the chamber interior surfaces, including the chamber walls and the susceptor. A typical cleaning gas system is a mixture of nitrogen trifluoride, hexafluoroethane and oxygen for cleaning unwanted silicon oxide films from the chamber interior. A plasma gas is typically ignited in the chamber to enhance the efficiency of the cleaning gas mixture. However, the reactive species of the cleaning gas cannot reach the throttle valve for effective cleaning due to the limited lifetime of the reactive species. Consequently, after multiple deposition and cleaning processes are performed in the chamber, a substantial amount of unwanted silicon oxide film is deposited and remains on the throttle valve, rendering it nonfunctional. That is, a sufficient amount of material is deposited on the interior surfaces of the throttle valve to prevent smooth motion of the throttle valve and accurate pressure control in the reaction chamber. This poor pressure control in the reaction chamber contributes to the production of semiconductor devices having insufficient reliability.  
         [0008]     In addition, deposited material which builds up on the throttle valve may become dislodged and travel back through the isolation valve and exhaust gas port, and into the reaction chamber. Semiconductor wafers subsequently processed in the CVD chamber will be exposed to this foreign material, which will negatively impact manufacturing yield.  
         [0009]     This problem of deposited material build-up on the throttle valve requires complete disassembly of the throttle valve assembly and manual cleaning by a wet chemistry technique. This is a very labor intensive and time consuming process which leads to poor throughput and increased cost of manufacturing. Moreover, after each manual disassembly and cleaning, the entire exhaust flow control system must be recalibrated in order to resume processing of semiconductor wafers in the reaction chamber.  
         [0010]     Furthermore, if the reaction chamber has become contaminated with foreign material which has been dislodged from the throttle valve, the entire chamber must be opened and cleaned manually through a similarly labor intensive process. Once the chamber cleaning has been completed, the entire CVD system must be recalibrated and requalified in order to resume processing of semiconductor wafers in the reaction chamber.  
         [0011]     An in-situ cleaning method and apparatus has been proposed by Robles et al. in U.S. Pat. No. 5,707,451. Robles et al. reposition the throttle valve assembly so that it is located upstream of the isolation valve and therefore closer to the exhaust gas port of the reaction chamber. Locating the throttle valve adjacent to the chamber increases the chance that some of the reactive species of the chamber cleaning gas may reach the throttle valve within their limited lifetime. However, this arrangement still suffers from reduced cleaning efficiency with regard to the throttle valve, because the throttle valve still is located relatively far from the plasma gas which is ignited in the chamber. Thus, most of the reactive species of the cleaning gas still do not reach the throttle valve for effective cleaning due their limited lifetime. More importantly, due to the absence of an isolation valve between the throttle valve and the reaction chamber, it is impossible in this arrangement to prevent material dislodged from the throttle valve, or any other foreign material in the CVD exhaust system, from contaminating the chamber.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention eliminates the aforementioned problems by providing an in-situ apparatus and method for effectively cleaning a throttle valve in a CVD system.  
         [0013]     In one aspect of the present invention, an exhaust flow control apparatus attached to a CVD chamber, for controlling an exhaust flow passage and for regulating gas pressure in said CVD chamber, is disclosed. The exhaust flow control apparatus comprises: an isolation valve in fluid communication with said CVD chamber, for opening and closing said exhaust flow passage; a throttle valve mounted downstream from and in fluid communication with said isolation valve, for regulating gas pressure in said CVD chamber; means for introducing a cleaning gas into said exhaust flow passage downstream of said isolation valve and upstream of said throttle valve; and a vacuum pump mounted downstream from and in fluid communication with said throttle valve, for evacuating gas from said CVD chamber. The apparatus optionally may further comprise means for applying RF power in said exhaust flow passage downstream of said isolation valve and upstream of said throttle valve, for generating a reactive plasma of said cleaning gas.  
         [0014]     In another aspect of the present invention, an exhaust flow control apparatus attached to a CVD chamber, for controlling an exhaust flow passage and a cleaning gas flow passage, and for regulating gas pressure in said CVD chamber, is disclosed. The exhaust flow control apparatus comprises: a first isolation valve in fluid communication with said CVD chamber, for opening and closing said exhaust flow passage; a second isolation valve in fluid communication with a cleaning gas source, for opening and closing said cleaning gas flow passage; a throttle valve mounted downstream from and in fluid communication with said first isolation valve and said second isolation valve, for regulating gas pressure in said CVD chamber; and a vacuum pump mounted downstream from and in fluid communication with said throttle valve, for evacuating gas from said CVD chamber. The apparatus optionally may further comprise an RF power source for generating a reactive plasma of said cleaning gas in said exhaust flow passage downstream of said isolation valve and upstream of said throttle valve.  
         [0015]     In yet another aspect of the present invention, a method for cleaning a throttle valve attached to a CVD chamber is disclosed. The method comprises the steps of: isolating said throttle valve from said CVD chamber; and flowing at least one cleaning gas into said throttle valve at a temperature and pressure and for a length of time such that unwanted film deposits are removed from said throttle valve. The method optionally may further comprise the step of generating a reactive plasma of said cleaning gas, prior to the step of flowing said cleaning gas into said throttle valve. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The drawings are for illustration purposes only and are not drawn to scale. Furthermore, like numbers represent like features in the drawings. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows, taken in conjunction with the accompanying drawings, in which:  
         [0017]      FIG. 1A  is a schematic view of a prior art exhaust flow control apparatus;  
         [0018]      FIG. 1B  is a schematic view of an exhaust flow control apparatus of the present invention; and  
         [0019]      FIG. 2  is a process flow diagram for a CVD chamber and exhaust flow control apparatus of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]      FIG. 1A  shows a typical prior art exhaust flow control apparatus attached to a CVD system, such as the Precision 5000 System available from Applied Materials, Inc., Santa Clara, Calif. A CVD reaction chamber  100  for processing semiconductor wafers has an exhaust flow control apparatus attached to the side of the chamber through a flow adapter  110 . Connected to the flow adapter  110  is a chamber isolation valve  111  for the opening and closing of the flow passage therein. Throttle valve  113  is connected to and in fluid communication with the chamber isolation valve  111  via exhaust pipe  112 . Throttle valve  113  is controlled by a precision servo-motor  114  which is in turn controlled by closed-loop feedback signals received from a pressure manometer (not shown) attached to the CVD chamber  100 . The gases exhausted from the CVD chamber  100  pass through flow adapter  110 , chamber isolation valve  111 , exhaust pipe  112  and throttle valve  113  into flow passage pipe  115  to a vacuum pump (not shown).  
         [0021]     In this prior art arrangement of the exhaust flow control apparatus, cleaning gases used in CVD chamber  100  for removing unwanted deposits from surfaces of the chamber interior must travel through flow adapter  110 , chamber isolation valve  111  and exhaust pipe  112  before reaching throttle valve  113 . A plasma gas typically is ignited in CVD chamber  100  to enhance the efficiency of the cleaning gas mixture. However, the reactive species of the cleaning gas cannot reach throttle valve  113  for effective cleaning due the limited lifetime of the reactive species. Consequently, after multiple deposition and cleaning processes are performed in chamber  100 , a substantial amount of unwanted film is deposited and remains on throttle valve  113 , rendering it nonfunctional. That is, a sufficient amount of material is deposited on the interior surfaces of throttle valve  113  to prevent smooth motion of the throttle valve and accurate pressure control in reaction chamber  100 .  
         [0022]      FIG. 1B  shows an improved exhaust flow control apparatus according to the present invention. In this embodiment, means are provided for introducing a cleaning gas downstream of chamber isolation valve  111  and upstream of throttle valve  1113 , by connecting a cleaning gas pipe  116  via a T-connection to exhaust pipe  112 , thereby forming a cleaning gas flow passage. A cleaning isolation valve  117  is installed in cleaning gas pipe  116 , for opening and closing the cleaning gas flow passage.  
         [0023]     While gases are exhausted from CVD chamber  100 , chamber isolation valve  111  remains open, and cleaning isolation valve  117  remains closed. Throttle valve  113  cycles between the open and closed positions as in a conventional CVD system in order to regulate the chamber pressure. Throttle valve  113  is controlled by servo-motor  114  which is in turn controlled by closed loop feedback signals received from a pressure manometer (not shown) attached to the CVD chamber  100 .  
         [0024]     When cleaning of throttle valve  113  is desired, chamber isolation valve  111  is closed, and cleaning isolation valve  117  is opened. Cleaning gases are introduced into cleaning gas pipe  116 , pass through cleaning isolation valve  117 , and enter throttle valve  113 . A plasma gas may be ignited by an RF power source (not shown) just before throttle valve  113 , for example in cleaning gas pipe  116  or in exhaust pipe  112 . Alternatively, a plasma gas may be ignited just before cleaning isolation valve  117 , so long as the distance to be traveled through cleaning isolation valve  117 , cleaning gas pipe  116  and exhaust pipe  112  is not excessive. Cleaning gases and byproducts then continue through flow passage pipe  115  to a vacuum pump (not shown).  
         [0025]      FIG. 2  shows a process flow diagram for a CVD process in which a CVD chamber  200  is used. Reactant gases  201  flow into chamber  200  through flow control valve  203 , gas inlet  204 , and gas distribution plate  205 . Gas inlet  204  and gas distribution plate  205  also act as the upper electrode for the RF source. Gas distribution plate  205  is sometimes called a showerhead. The lower electrode or susceptor  206  is normally grounded when RF power is required. A RF generator (not shown) may provide RF power  202  through a matching network (not shown) to the upper electrode (gas inlet  204  and gas distribution plate  205 ). A pressure manometer  207  monitors the gas pressure in chamber  200 .  
         [0026]     There are a number of different types of thin films that can be deposited using CVD. The reactant gases to be used, and the chamber pressure and temperature, vary depending on the type of thin film desired. For silicon oxide films, the reactant gases may include tetraethoxyorthosilicate (TEOS), optionally with a carrier gas such as helium, oxygen (O 2 ), and ozone (O 3 ), or silane (SiH 4 ) and nitrous oxide (N 2 O). The chamber pressure may be maintained at between about 40 torr and about 600 torr during the deposition of silicon oxide films, or may be maintained as low as about 8 torr for plasma-enhanced CVD. The temperature of the chamber is elevated to usually greater than 100° C. At this elevated temperature, and if desired, with RF applied, the gases will react and deposit a silicon oxide layer on the surface of the wafer.  
         [0027]     During the deposition process, chamber isolation valve  211  remains open and cleaning isolation valve  217  remains closed. Gases from the reaction chamber  200  are exhausted through chamber isolation valve  211  and throttle valve  213 , to a vacuum pump (not shown). Throttle valve  213  cycles between the open and closed positions to regulate the gas pressure in chamber  200 . The position of throttle valve  213  is controlled by a servo-motor (not shown) which is in turn controlled by a closed-loop control system based on feed-back signals from pressure manometer  207 .  
         [0028]     Reactant gases deposit a film not only on the semiconductor wafer, but also on all of the interior surfaces of chamber  200 , as well as on throttle valve  213 . When the deposition process is completed, the wafer is removed from the chamber and a cleaning process is performed to remove deposits from the walls of the chamber. For the chamber clean, cleaning gases  201  are flowed into the chamber  200  through gas inlet  204  and gas distribution plate  205 .  
         [0029]     For cleaning following a silicon oxide film deposition, nitrogen trifluoride (NF 3 ), hexafluoroethane (C 2 F 6 ) and oxygen (O 2 ) may be used. The flow rate of the cleaning gases is controlled such that the chamber pressure can be maintained at usually less than 200 torr. The temperature inside chamber  200  is maintained between about 100° C. to about 500° C. A plasma is ignited in the cleaning gas by applying RF power  202 , thereby causing the gas to react with the deposit layers and etch the layers away. RF power of about 700 watts to about 1500 watts, usually about 900 watts, may be applied.  
         [0030]     During the chamber cleaning process, cleaning gases are exhausted through chamber isolation valve  211  and throttle valve  213  to a vacuum pump (not shown). Chamber isolation valve  211  is in the open position, and cleaning isolation valve  217  is in the closed position.  
         [0031]     Either before the chamber cleaning process is begun or after the chamber cleaning process is completed, the throttle valve cleaning process of the present invention may be commenced. Chamber isolation valve  211  is closed, and cleaning isolation valve  217  is opened. Before cleaning gases are introduced through cleaning isolation valve  217 , purge gases  221  may be flowed through cleaning isolation valve  217 , into the exhaust pipe upstream of throttle valve  213 , and through throttle valve  213 . Purge gases may be inert or “house” gases, such as oxygen (O 2 ) or nitrogen (N 2 ) or a mixture of these gases. Purge gases may be flowed at a rate of as much as about 5 standard liters per minute (slm), for as long as about 1 minute.  
         [0032]     Cleaning gases  221  are then flowed through cleaning isolation valve  217 , into the exhaust pipe upstream of throttle valve  213 , and through throttle valve  213 . The same cleaning gases used to clean the chamber may be used to clean the throttle valve, or different cleaning gases may be used. For example, when cleaning a throttle valve following a silicon oxide film deposition, nitrogen trifluoride (NF 3 ), hexafluoroethane (C 2 F 6 ) and oxygen (O 2 ) may be used. Alternatively, fluorine (F 2 ) may be used, at a flowrate of about 1 slm for about 20 seconds, depending on the deposited film thickness.  
         [0033]     The pressure in the piping between chamber isolation valve  211  and throttle valve  213  should be maintained in the range of about 20 mtorr to about 10 torr. This may be accomplished by reducing the flow of gases  221 , and/or by cycling throttle valve  213  between the open and closed positions via a servo-motor (not shown) controlled by a closed-loop control system based on feed-back signals from a pressure manometer (not shown) installed between chamber isolation valve  211  and throttle valve  213 . The pressure is preferably measured and stabilized using purge gases, prior to introducing cleaning gases.  
         [0034]     The piping between chamber isolation valve  211  and throttle valve  213  need not be heated or cooled during the cleaning method of this invention. However, heating of the piping between chamber isolation valve  211  and throttle valve  213  may enhance the effectiveness of the cleaning gases.  
         [0035]     While cleaning gases  221  are being introduced through cleaning isolation valve  217 , a plasma may be ignited in the cleaning gas by applying RF power  222 , thereby causing the gas to react with the deposited material and etch the material away. The plasma may be generated using any conventional means. For example, a remote RF source may be used, which would require much less power than the chamber RF source. For example, a remote RF source having a power as low as about 5 watts, up to about 1500 watts, may be used for the throttle valve cleaning process. Preferably, an inductive plasma system may be employed to generate plasma for the throttle valve cleaning process. In  FIG. 2 , RF power is shown being applied in the exhaust flow passage downstream of chamber isolation valve  211  and upstream of throttle valve  213 . However, RF power may also be applied in the cleaning gas passage downstream of cleaning isolation valve  217 , or even upstream of cleaning isolation valve  217 , so long as the distance to be traveled through cleaning isolation valve  217  and to throttle valve  213  is not excessive.  
         [0036]     Some types of throttle valves may need to be actuated or rotated while the reactive plasma is being generated. For example, certain vales, such as the MKS throttle valve or the Applied Materials (AMAT) sigma throttle valve, should be repositioned during generation of the reactive plasma in order to effectively clean all surfaces of the valve. Other types of valves, such as a C-plug valve or a dual spring valve, need not be actuated during reactive plasma generation.  
         [0037]     After the throttle valve cleaning process is complete, the remaining cleaning gases and any cleaning byproducts are pumped out of the piping and the throttle valve. Optionally, an inert gas may be used to purge the remaining cleaning gases and cleaning byproducts.  
         [0038]     While the present invention has been particularly described in conjunction with a preferred embodiment and other alternative embodiments, it is evident that numerous alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore intended that the appended claims embrace all such alternatives, modifications and variations as falling within the true scope and spirit of the present invention.