Abstract:
A system for controlling the flow of gases into a reaction chamber used in processing semiconductor devices includes a safety interlock feature that prevents inadvertent mixing of incompatible, reactive gases. The interlock feature is implemented in an interlock control circuit which operates a valve system for individually controlling the flow of separate gases into the chamber. The interlock circuit includes a series of relay switches and timers arranged to create a time delay between the initiation of flow of gases from separate sources into the chamber.

Description:
TECHNICAL FIELD 
     The present invention broadly relates to methods and equipment used to manufacture semiconductor devices, and deals more particularly with a system for controlling the flow of incompatible reactive gasses into a semiconductor processing chamber. 
     BACKGROUND OF THE INVENTION 
     In connection with manufacturing processes for producing semiconductor devices, a variety of techniques and processes are used which require the use of reactive gasses. One such common process is chemical vapor deposition (CVD) which is a thin film growth process wherein very high quality films are deposited onto a heated substrate. CVD processes are widely used for forming various layers during integrated circuit fabrication. For example, CVD processes have long been used for deposition of polysilicon, tungsten, silicon nitride, silicon oxynitride and various forms of silicon dioxide. CVD processes are also coming into use for deposition of conductive materials such as aluminum, metal sicilicides, and titanium nitride. CVD processes generally involve a decomposition of a precursor gas mixture, at the surface of the heated substrate, to form components which are the chemical precursors of the desired film composition. For example, polysilicon can be grown by decomposition of dichlorosilane. The CVD process is carried out in a sealed container which is evacuated before the selective introduction of reactive gases. These reactive gases are typically introduced in a serial manner to effect successive process steps. Typically, the reactive gases are delivered to the chamber from individual sources through a series of conduits and valves, which, in many cases result in each of the gases flowing through a single conduit that feeds the gas into the chamber. In many cases, the gases, if mixed, are potentially explosive. In other cases, the admixing of a small amount of residual gas from a prior process into the mainstream of gas flowing into the chamber at the beginning of a second process may result in contamination of the chamber which in turn affects the process and the ultimate quality of the semiconductor devices produced thereby. 
     Thus, there is a clear need in the art to provide better flow control of reactive, process gases into processing chambers in order to reduce the possibility that even small amounts of incompatible gases may be mixed together or simultaneously introduced into a processing chamber such that the chamber becomes contaminated. The present invention is intended to satisfy this need. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a system is provided for controlling the introduction of chemically reactive gasses into a processing chamber used to manufacture semiconductor devices. The system broadly comprises a pair of valves systems for respectively controlling the flow of first and second reactive gases into the chamber, and a cross interlock circuit coupled with and controlling the valve systems for preventing simultaneous flow of the first and second gases into the chamber. The interlock circuit separately controls the valve systems so as to prevent one valve system from opening until the other valve system has been closed. Additionally, the interlock circuit includes a time delay feature which the delays the opening one valve system until a pre-selected time after the other valve system has been closed. This time delay assures that reactive gases flowing into the chamber from a recently closed valve system will not inadvertently mix with gas allowed to flow into the chamber when the other valve system is opened. Each of the valve systems includes a first, electrically operated valve controlled by the interlock circuit, and a second pneumatically controlled valve operated by the first valve. The pneumatic valve controls the flow of reactive gas from a source into the processing chamber. The interlock circuit may be implemented using discrete relays and timers, or by employing a programmable logic controller. 
     According to another aspect of the invention, a method is provided for controlling the introduction of chemically reactive gases into a processing chamber used to manufacture semiconductor devices, comprising the steps of: producing first and second control signals for respectably opening first and second valve systems that allow the flow of first and second reactive gases into a processing chamber; delaying the opening of the first valve system for a pre-determined length of time after the first control signal is produced; and, delaying the opening of the second valve system for a second predetermined length of time after the second control signal is produced. The steps of delaying the opening of the first and second valve systems prevents potentially hazardous mixing of the first and second reactive gases. The delays are performed by activating timers to establish a time count corresponding to the predetermined time delays, and delivering the respective control signals to the corresponding valve systems only after the time count reaches the pre-determined time delay values. 
     Accordingly, it is the primary object of the present mentioned to provide a system for controlling the flow of reactive gases into a processing chamber which precludes potentially hazardous mixing of the gases. 
     Another object of the invention is to provide a system as described above which the limits the possibility of small amounts of residual gases from entering or remaining in the processing chamber at the commencement of a new process step. 
     A still further object of the present pension is to provide a system of the type described above which utilizes an interlock control circuit utilizing conventional control components. 
     Another object of the invention is to provide a system as described above which includes a two tiered valve system in which control signals produced by the interlock circuit result in the operation of pneumatically operated valves that control the flow of reactive gases into the chamber. 
     These, and further objects and advantages of the present invention will be made clear or will become apparent during the course of the following description of a preferred embodiment of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings which form an integral part of the specification and are to be read in conjunction therewith, and in which like reference numerals are employed to designate identical components in the various views: 
         FIG. 1  is a combined block and diagrammatic view of a system for controlling the flow of gas into a semiconductor processing chamber; and, 
         FIG. 2  is a ladder logic diagram of the cross interlock circuit shown in  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to  FIG. 1 , the present invention generally relates to a system for controlling the flow of reactive gasses from a source thereof into a processing chamber  10 , wherein semiconductor manufacturing processes, such as CVD operations are performed. In the illustrated embodiment, O 2  gas is derived from a first, valve manifold box (VMB)  12 , and a gas mixture of N 2  and H 2  is derived from a second valve manifold box  14 . The O 2  gas from VMB  12  is passed through a manually operated valve  20  to a mass flow controller (MFC)  24 . Similarly, the N 2 /H 2  gas mixture derived from VMB  14  is passed through a second manually operated valve  22 , into a second MFC  26 . 
     The VMB&#39;s  12 ,  14  comprise conventional, gas handling boxes, also sometimes referred to as gas isolation boxes, which include an enclosure into which gas handling equipment and/or a source of gas contained in a cylinder are housed. Typically these housings are ventilated and configured to protect the surrounding environment from potentially corrosive or hazardous gasses. The MFC&#39;s  24 ,  26  are also conventional devices well known in the art which are typically used to introduce a specific amount of gas flow for a particular gas species into a reaction chamber so that the appropriate pressure and/or flow rates of gas are achieved. The MFC&#39;s  24 ,  26  may incorporate flow sensors that are calibrated for the corresponding gas and function to determine the flow rate of the gas. The sensor information may also be used in combination with an electronic control system (not shown) to alter actuator settings, such as valves  20 ,  22  in order to control gas flow. 
     Gas flows from the MFC&#39;s  24 ,  26  respectively through a pair of metering type valves  28 ,  30  which typically include a variable orifice for selectively varying the rate at which gas flows therethrough. In effect, valves  28 ,  30  function as flow restrictors which effectively control the rate at which the chamber  10  becomes pressurized with gas. One suitable commercial version of valve  28 ,  30  is known as a NuPro valve which is well known in the industry. Normally, the orifice size of valve  28 ,  30  are selected to provide as fast as possible pressure equalization within the chamber  10  without degrading the quality of the semiconductor wafer being processed in the chamber  10 . Valves  28 ,  30  are each operated by a pressurized fluid such as compressed dried air. 
     The gases metered through the valves  28 ,  30  flow through a single final valve  32  before entering the chamber  10 . Chamber  10 , as previously mentioned, may comprise a conventional reaction chamber in the form of a metal vessel in which chemical reactions can be carried out during the processing of semiconductor wafers. The chamber  10  is leak tight so that low pressures can be achieved for processing, and must be able to withstand the introduction of chemically reactive gasses, elevated temperatures of the wafer, and plasma discharges internal to the chamber. 
     The metering valves  28 ,  30  are controlled by a pair of respectively associated, electromagnetically controlled valves  34 ,  36 . Valves  34 ,  36  are respectively coupled with a source of compressed dried air delivered by lines  50 ,  52 . The operation of valves  34 ,  36  are controlled by electrical control signals derived on lines  42 ,  44  from a cross interlock circuit  18 . A control signal on line  42  thus actuates valves  34  to either open or closed close. In the open position, valves  34  allows compressed dried air from line  50  to flow through line  38  to the metering valve  28  to either open our close the latter. In a similar manner, electrical controls signals on line  44  causes valve  36  to either open or close. When in the open position, valve  36  allows compressed dried air to flow from line  52  through line  40  to metering valve  30 , thereby actuating the latter. 
     From the foregoing description, it can be appreciated that reactive gasses are derived from sources thereof and are delivered into the chamber  10  through a series of valves, some of which are common to both process gases. Although gases from only two sources  12 ,  14  are shown in  FIG. 1 , it is to be understood that with the provision of additional valves and conduits, process gasses may be derived from multiple other sources. In connection with chemically reactive processes that are carried out in the chamber  10 , reactive gasses are successively delivered into the chamber  10  in order to carry our successive process steps. In some cases, the reactive gases used in different, successive process steps may react with each other if inadvertently mixed together, which could then result in a hazardous reaction or even an explosion. Such inadvertent mixing could occur if small amounts of one gas remain in certain of the delivery lines or within the chamber  10  while a second gas required for a subsequent step is introduced into the chamber  10 . 
     This potentially hazardous situation may occur because a short amount of time is required after a valve is actuated to cut off the flow of gas before that gas is purged from the delivery lines and chamber  10 ; if a different reactive gas is introduced into the delivery lines and the chamber  10  too quickly, dangerous mixing of the two gasses can occur. 
     In order to prevent potentially hazardous mixing of reactive gasses, and accordance with the present invention, a novel cross interlock circuit  18  is provided, the details of which are shown in  FIG. 2 .  FIG. 2  is a ladder logic diagram of the circuit  18 , and it should be understood that the circuit shown therein may be implemented either in the form of discrete components such as relays or timers, or by a PLC (programmable logic controller).  FIG. 2  depicts the circuit in discrete component form in which the components are coupled with a 24 volt power source. 
     Broadly, the circuit  18  comprises a pair of relays  46 ,  48  and relay timers  54 ,  56  which all cooperate to control the metering valves  34 ,  36  in a manner which provides both an interlock function and a delay function. These two functions not only preclude two different gasses from being introduced into the chamber  10  at the same time, but also provide for a slight time delay between the stoppage of a flow of one gas, and the commencement of flow of another gas required in a subsequent processing step. This time delay is pre-selected to provide sufficient time to allow residual gas from a previous process to flow through the delivery lines and out of the chamber  10  before the next gas is allowed to flow into chamber  10 . 
     Relay  46  has a coil that is activated by a first valve signal commanding the flow of the N 2 /H 2  gas mixture. The coil of relay  46  controls a set of normally closed relay contacts  60  which are coupled in series with relay timer  56  and the power source. The coil of relay  46  also operates a second set of normally open contacts  64  which are coupled in series with relay contacts  68 , the coil of valve  36  and the power source. Similarly, relay  48  includes a coil that is actuated by a signal commanding the flow of the O 2  gas. The coil of relay  48  controls a normally closed set of relay contacts  58 , and a normally open set of contacts  62 . Relay contacts  58  are coupled in series with the relay timer  54  and the power source. Contacts  62  are coupled in series with relay timer contacts  66 , the power source and the actuating coil of metering valve  34 . Contacts  66  are controlled by the relay timer  54 , while contacts  68  are is controlled by the relay timer  56 . 
     In operation, a control signal delivered to relay  46  commanding the commencement of flow of the N 2 /H 2  gas mixture energizes the coil of relay  46  causing the normally closed relay  60  to open and the normally closed relay  64  contacts to close. At this point, relay  48  is de-energized, consequently normally closed contacts  58  remain closed and relay timer  54  is coupled with the power source causing contact  68  to close, thus coupling the electromagnetic valve  36  with the power source. With the coil of valve  36  energized, this valve is opened, thereby placing the compressed dried air in line  52  in communication with air line  40 . With air line  40  pressurized, metering valve  30  is opened, thereby allowing the N 2 /H 2  gas mixture to flow from the MFC  26  into the chamber  10 . For sake of simplification, the final valve  32  is not shown in  FIG. 2 . 
     When the command signal is removed from relay  46  and a command signal is delivered to relay  48  in order to initiate flow of the O 2  gas, the following occurs. Deactivation of relay  46  closes contacts  60  and opens contacts  64 . Energizing the coil of relay  48  results in contacts  58  opening and contacts  62  closing. However, even though contacts  62  close, contacts  66 , controlled by relay timer  56 , remain open for a short period of time, for example 0.5 seconds, until relay timer  56  times out. When relay timer  56  reaches a preselected count and times out, contacts  66  are closed, thereby coupling the valve  34  with the power source which results in the closing of the valve  34  and subsequent flow of the O 2  gas to the chamber  10 . 
     When relay  48  is switched off and relay  46  is turned back on causing contacts  64  to close, relay timer  54  holds contacts  68  open for the pre-determined length of time to allow the N 2 /H 2  gas mixture to flow out of the lines and the chamber  10 . After relay timer  54  times out, contacts  68  are closed, thereby coupling the power supply with the actuating coil of relay  36 . 
     It is thus apparent that the cross interlock circuit cooperates with a pair of valve systems to prevent inadvertent mixing of two reactive gasses. Relay  46  and contacts  60  form a first actuatable relay circuit for operating one of the valve systems, and relay  48  and its associated contacts  62  function as a second actuatable relay circuit for controlling the operation of the second valve system. Relay timer  56  and its associated contacts  66  function as a first time delay controller for delaying the operation of the fist valve system for a pre-determined length of time after the first relay circuit has been actuated in order to assure that the flow of one gas into the chamber has stopped before the second gas begins to flow. Similarly, relay timer  54  and its associated contacts  68  function as a second time delay controller for delaying the operation of the second valve system for a pre-determined period of time after the second relay circuit has been actuated in order to assure that the flow of the second gas into the chamber has stopped before the first gas begins to flow into the chamber. It may be further appreciated that a method is provided for controlling the introduction of chemically reactive gasses into a processing chamber used to manufacture semiconductor devices which comprises the steps of producing a first control signal for opening a first valve system allowing the flow of a first reactive gas into a chamber; producing a second control signal for opening a second valve system allowing the flow of a second reactive gas into the chamber; delaying the opening of the first valve system for a pre-determined length of time after the first signal is produced; and, delaying the opening of the second valve system for a second pre-determined length of time after the second signal is produced. 
     From the foregoing, it is apparent that the system of the present invention not only provides for the reliable accomplishment of the objects of the invention, but does so in a particularly simple and economic manner. It is recognized, of course, that those skilled in the art may make various modifications or additions chosen to illustrate the invention without departing from the spirit and scope of the present contribution to the art. Accordingly, it is to be understood that the protection sought and to be afforded hereby should be deemed to extend to the subject matter claimed and all equivalents thereof fairly within the scope of the invention.