Abstract:
A method and system are provided for delivering a source gas to a processing chamber. A source gas delivery method includes providing a precursor chamber configured to hold precursor vapor, providing a saturated precursor vapor at a selected pressure within the precursor chamber, and flowing or diffusing saturated precursor vapor from the precursor chamber to the processing chamber until a selected pressure is provided in the processing chamber. A source gas delivery system includes a precursor chamber configured to hold precursor vapor, a heat source for heating a precursor liquid to provide saturated precursor vapor at a selected pressure within the precursor chamber, and a vapor pathway allowing saturated precursor vapor to enter a processing chamber until a selected pressure is provided in the processing chamber. Advantageously, the present invention allows for improved precursor vapor delivery and enhanced control over thin film deposition with less waste of precursor material.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention generally relates to semiconductor manufacturing equipment and, more particularly, to the controlled delivery of source gas. 
     2. Description of Related Art 
     Advanced thin film materials are increasingly important in the manufacture of microelectronic devices. In contrast to traditional thin films, future thin films require new source materials that have low vapor pressures and that are often near their decomposition temperature when heated to achieve an appropriate vapor pressure. Some of the precursors, having both intrinsically low vapor pressure and low thermal decomposition temperature, are considered the best choices for deposition of films of tantalum oxide, tantalum nitride, titanium nitride, copper, and aluminum. For such applications, it is essential that the film morphology and composition be closely controllable. This in turn requires highly reliable and efficient means and methods for delivery of source reagents to the area of film formation. 
     In some cases, the delivery of reagents into the reactor in the vapor phase has proven difficult because of problems of premature decomposition or stoichiometry control. Examples include the deposition of tantalum oxide from the liquid precursor tantalum pentaethoxide (TAETO) and the deposition of titanium nitride from bis(dialkylamide)titanium reagents. 
     A precursor is the source of a vapor to be used in forming a thin film. Additionally, precursors often contain impurities, and the presence of those impurities can cause undesirable thermally activated chemical reactions at the vaporization zone, also resulting in formation of involatile solids and liquids at that location. For example, a variety of precursors, such as TAETO, are water-sensitive and hydrolysis can occur at the heated vaporizer zone forming tantalum oxide particulates that may be incorporated into the growing tantalum oxide film with deleterious effects. 
     Various source reagent delivery systems have been commonly employed to introduce vapors of source reagents to chemical vapor deposition (CVD) reactors. These include bubbler-based systems, liquid mass flow control systems, and liquid metering by pump systems. The use of CVD reactors is well known for thin film deposition and other thermal process steps required in the manufacture of integrated circuits. 
     FIG. 1A illustrates a typical bubbler-based delivery system, which includes an enclosed precursor chamber  10  at least partially submerged in the liquid of a heating bath  20 . The temperature of the bath may be adjusted to heat or cool precursor chamber  10 . In operation, precursor chamber  10  contains a precursor liquid. An inert carrier gas travels to precursor chamber  10  along a first pipe  30 . The open end of first pipe  30  is located in the precursor liquid. The carrier gas exits the pipe and bubbles to the surface of the precursor liquid. Contained within precursor chamber  10  above the surface of the precursor liquid is a space  40 . An input end for a second pipe  50  is located in space  40  above the surface of the precursor liquid. As the stream of the inert gas passes through the liquid precursor and bubbles to the liquid surface, precursor vapor attains its equilibrium vapor pressure more quickly. A “sparger” (a cap with multiple small perforations) is sometimes added to the end of first pipe  30  to ensure formation of small bubbles and rapid equilibration. The carrier gas and precursor vapor enter second pipe  50  and flow to a processing chamber, where the precursor vapor reacts upon a surface of a heated substrate. The temperature of pipe  50  is controlled by heating elements, such as heating coils  55 , surrounding second pipe  50  to keep the precursor vapor from condensing during transport to the processing chamber. 
     The performance of bubbler-based delivery systems is complicated by the exponential dependence of liquid vapor pressure on temperature. Small changes in temperature can cause large changes in reagent delivery rate, leading to poor process control. Lower temperatures and/or higher flow rates of the bubbled carrier gas tend to lower vapor pressure. Therefore, fluctuations in carrier gas temperature and flow rate can cause the vapor pressure of the precursor liquid to fluctuate. Accordingly, the precursor vapor will not always be saturated, leading to fluctuating concentrations of the source reagent. Further, vapor concentration in the bubbler-based system is a function of carrier gas contact time in the fluid as the carrier gas bubbles to the surface. Thus, vapor concentration fluctuates over time as the level of precursor liquid in the precursor chamber changes with use. 
     FIG. 1B illustrates another common delivery system using a liquid mass flow controller (LMFC) to measure and control the flow rate of liquid precursor to a vaporizer. An enclosed precursor chamber  10  includes a precursor liquid. An inert gas travels to precursor chamber  10  along a first pipe  30 . The open end of the pipe is located above the surface of the precursor liquid. Inert gas exits first pipe  30  and pressurizes the precursor liquid within precursor chamber  10 . An input end for second pipe  50  is located in the precursor liquid. When the inert gas enters precursor chamber  10 , the space above the precursor liquid becomes pressurized such that the level of the precursor liquid within precursor chamber  10  is lowered. Precursor liquid enters second pipe  50  and is transported to a LMFC  60 . The precursor liquid exits LMFC  60  and is transported to a vaporizer  70 . The precursor liquid is vaporized and is then typically entrained in a carrier gas which delivers it to the heated substrate. Gas exits the vaporizer through a heated pipe  90 . The temperature of the pipe is controlled by heating elements, such as heating coils  95 , surrounding the pipe. 
     Disadvantageously, liquid mass flow controllers present a number of drawbacks. LMFCs are extremely sensitive to particles and dissolved gases in the liquid precursor. LMFCs are also sensitive to variations in the temperature of the liquid precursor. Further, most LMFCs cannot operate at temperatures above 40° C., a temperature below which some precursor liquids, such as TAETO, have high viscosity. Another drawback with LMFC-based systems can be attributed to the “dead volume” (e.g. piping) between the LFMC and the vaporizer. Any amount of liquid that remains in the dead volume can contribute in making an inaccurate delivery of source reagent to the vaporizer. These drawbacks can make volumetric control of the liquid precursor very difficult. 
     The aforementioned inaccuracies in volumetric control of the liquid precursor cause large inaccuracies in final delivery of the precursor vapor to the processing chamber since a small variation in liquid volume (flow rate) results in a large variation in gas volume (flow rate). Further, LMFC-based systems typically use a gas to assist in the vaporization of the liquid precursor, thereby increasing the probability of generating solid particles and aerosols. Additionally, spatial and temporal temperature variations usually occur in the vaporization zone, leading to inconsistent delivery of source reagents. 
     Finally, because of the temperature difference between the vaporizer and the pipe leading to the processor, such as pipe  90  in this example, condensation may occur during transport of the precursor vapor, which also contributes to inaccurate delivery of the source reagent. 
     FIG. 1C illustrates another well-known system using a pump for liquid metering of the precursor liquid to a vaporizer. Pump  80  pulls precursor liquid from precursor chamber  10  to a vaporizer  70 . Vapor exits the vaporizer in a heated pipe  90 . The temperature of the pipe is controlled by heating elements, such as heating coils  95  surrounding the pipe, to prevent condensation. 
     Pump-based systems have similar disadvantages as LMFC-based systems. Spatial and temporal temperature variations usually occur in the vaporization zone, leading to inconsistent delivery of source reagents. This system is also extremely sensitive to particles and gas dissolved in the liquid. Further, any dead volume of precursor liquid delivered to the vaporizer will increase inaccurate delivery of source gas. Thus, delivery of precursor vapor to the processing chamber lacks high accuracy in this system since a small error in liquid volume measurement or control leads to a large error in vapor volume. Finally, most pumps cannot tolerate high temperatures (maximum 50° C.), below which some precursor liquids have high viscosity. 
     Therefore, what is needed is a precursor vapor delivery method and system with control for precise stoichiometry by limiting fluctuations in the concentration of source gas delivered to the processing chamber. 
     SUMMARY 
     In accordance with the present invention, a method and a system are provided for the controlled delivery of source gas to a processing chamber. A source gas delivery method includes providing a precursor chamber configured to hold precursor vapor, providing saturated precursor vapor at a selected pressure within the precursor chamber, and flowing saturated precursor vapor from the precursor chamber to a processing chamber until a selected pressure is provided within the processing chamber. Advantageously, the present invention provides and controls precise stoichiometry involved in the process reactions by delivering accurate amounts of precursor vapor to the processing chamber. 
     In another aspect of the present invention, a source gas delivery method includes providing a precursor chamber configured to hold precursor vapor, providing saturated precursor vapor at a selected pressure within the precursor chamber, and diffusing saturated precursor vapor from the precursor chamber to a processing chamber until a selected pressure is provided within the processing chamber. 
     In yet another aspect of the present invention, a source gas delivery system includes a precursor chamber configured to hold precursor vapor, a heat source for heating a precursor liquid to provide saturated precursor vapor at a selected pressure within the precursor chamber, and a vapor pathway allowing saturated precursor vapor to enter a processing chamber until a selected pressure is provided in the processing chamber. 
     In FIG. 4, an advantage of the present invention is illustrated using graph  400 , which shows how the concentration of the vapor delivered to a processing chamber can vary over time. In a typical vapor delivery system, precursor vapor is delivered at a fairly constant level  402 , with fluctuations in the concentration occurring constantly. This manner of precursor vapor delivery ensures that enough source gas is available during processing operations. Unfortunately, most of the source gas is wasted since only a small percentage is consumed in the processing operation. The wasted gas must be vented, which can also require special procedures and additional treatments. 
     In the present invention, once a predetermined concentration of vapor  404  is delivered to the process chamber, the delivery is stopped. As the vapor reagents are consumed in the processing, the concentration level  404  drops, but the amount of wasted gas is substantially reduced since the difference between concentration level  402  and concentration level  404  will not have to be purged. 
     These and other features and advantages of the present invention will be more readily apparent from the detailed description of the embodiments set forth below taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a simplified illustration of a bubbler-based delivery system. 
     FIG. 1B is a simplified illustration of a liquid mass flow control delivery system. 
     FIG. 1C is a simplified illustration of a liquid metering by pump delivery system. 
     FIG. 2A illustrates a simplified schematic view of a source gas delivery system in accordance with an embodiment of the present invention. 
     FIG. 2B illustrates another simplified schematic view of a source gas delivery system in accordance with an embodiment of the present invention. 
     FIG. 3 is a graph of vapor pressure versus temperature for TAETO. 
     FIG. 4 is a graph of source gas concentration versus time in a processing chamber. 
     FIG. 5 is a flow chart of several source gas delivery methods in accordance with embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 2A illustrates source gas delivery system  300  in accordance with an embodiment of the present invention. A precursor liquid  302  is enclosed in a precursor chamber  304 . Precursor liquid  302  can include liquid reagents having low vapor pressure at room temperature, such as tantalum pentaethoxide (TAETO), triethylaluminum (TEA), trimethylaluminum (TMA), triethlyphosphorous (TEP), and triethylgallium (TEGa) or any other liquid source, such as SiCl 4 , GeCl 4 , HCl and the like. Precursor liquid  302  is not highly volatile but can be heated to form non-negligible precursor vapor. 
     Precursor chamber  304  is in thermal contact with a heat source  306  to heat precursor liquid  302 . Heat source  306  can be any heating apparatus which uniformly heats and controls the temperature of precursor liquid  302 , such as a heating bath, heating plate, and convection oven. In the embodiment illustrated in FIG. 2A, a temperature-controlled liquid bath  308  is used to heat precursor chamber  304 . Precursor chamber  304  is at least partially submerged in liquid bath  308  to a level where precursor liquid  302  is at least fully submerged in the bath fluid. In an alternative embodiment, precursor chamber  304  is fully submerged in the liquid bath to allow precursor vapor as well as the precursor liquid to be heated. In this illustrative embodiment, the liquid bath is heated to between approximately 50° C. and approximately 220° C. 
     Bath fluids having low volatility, high boiling points, and/or high heat capacities which can be used in liquid bath  308  are available commercially. Examples of bath fluids, with no intention to limit the invention thereby, are the Silicone series of bath fluids, available from Cole-Parmer Instrument Co., Vernon Hills, Ill. 
     Precursor chamber  304  includes a control diameter D 1 . As D 1  is made larger, the surface area of exposed precursor liquid  302  is increased. Accordingly, saturated precursor vapor is more quickly formed and made available for delivery to the processing chamber upon heating. 
     Control diameter D 1  also controls for backflow or negative pressure drop during vapor delivery to processing chamber  340 , which includes a diameter D 2 . For example, as control diameter D 1  is made larger relative to diameter D 2 , the pressure drop between precursor chamber  304  and processing chamber  340  becomes negligible, thereby controlling for backflow during vapor delivery. In one embodiment, control diameter D 1  is in the range of between approximately 25 mm and approximately 300 mm, and diameter D 2  is in the range of between approximately 50 mm and approximately 1000 mm. 
     Optionally, precursor chamber  304  is operably connected to a precursor source  309 . Precursor source  309  may continuously feed precursor liquid to precursor chamber  304  or it may feed discrete amounts of precursor liquid as needed. In the alternative, precursor chamber  304  is a stand-alone batch chamber that is manually refilled with precursor liquid as needed. 
     The source gas delivery system of the present invention further includes a vapor pathway allowing saturated precursor vapor to enter the processing chamber from the precursor chamber. In one embodiment, the vapor pathway includes a vapor inlet  320  located in a space  330  above the surface of liquid precursor  302  in precursor chamber  304 . A first end of pipe  322  is operably connected to vapor inlet  320 . A second end of pipe  322  is operably connected to open/close valve  324 . A first end of pipe  326  is also operably connected to open/close valve  324 , and a second end of pipe  326  is operably connected to processing chamber  340 . Valves and seals which can be used in this system are available commercially from Rohm and Haas Company, North Andover, Mass. 
     As shown by the flowchart in FIG. 5 in conjunction with system  300  in FIG. 2A, the source gas delivery method of the present invention includes the selection of a desired pressure of precursor vapor required for processing in processing chamber  340 . By having accurate control over the concentration of precursor vapor delivered to the processing chamber, improved control over stoichiometry, and therefore film composition, is achieved. The stoichiometry for the reaction involving precursor vapor and reactant gases in processing chamber  340  is determined by controlling the total pressure of processing chamber  340  and by controlling the temperature of heat source  306 , which determines the vapor pressure of precursor liquid  302 . 
     Dalton&#39;s law of partial pressures states that for a mixture of gases in a container, the total pressure exerted is the sum of the pressures that each gas would exert if it were alone. The pressure that each gas would exert if it were alone in the container is known as the partial pressure of each gas. Mole fraction, χ, of a particular component in a mixture, assuming ideal gases, is directly related to its partial pressure, being defined as partial pressure, Pn, divided by total pressure, Ptotal. Mole percentage of a particular component in a mixture is defined as mole fraction multiplied by 100. Thus, in the present invention, reaction stoichiometry and film composition are accurately controlled based upon the precise mole percentage or partial pressure of precursor vapor delivered to processing chamber  340 . 
     In accordance with the present invention, the selected pressure is the desired precursor vapor partial pressure needed for processing in processing chamber  340 . To generate the selected pressure, precursor liquid  302  is subjected to a temperature which correlates to the selected precursor vapor pressure. Thus, another step in the source gas delivery method of the present invention is to correlate a temperature to the selected precursor vapor pressure. The correlations between temperature and vapor pressure for pure substances are well known in the art. For example, FIG. 3 is a graph showing the vapor pressure of TAETO corresponding to temperature. Similar graphs exist for other substances. 
     Referring again to FIG. 5, a further step in the source gas delivery method of the present invention is to continually heat the precursor liquid at the determined temperature until the precursor liquid/vapor mixture reaches equilibrium. Accordingly, the precursor vapor will be saturated at a vapor pressure equal to the selected pressure. A pressure sensor  318  is optionally used to monitor precursor vapor pressure in precursor chamber  304  (FIG.  2 A). 
     After heating the precursor liquid at the correlated temperature to form saturated precursor vapor, alternative paths may be taken in the source gas delivery method of the present invention, as shown by FIG. 5 in conjunction with FIG.  2 A. Processing chamber  340  may be either vacuumed by a vacuum pump  350  or filled with reactant gases to a preselected pressure before receiving the precursor vapor. In one embodiment, as shown by path  500 , processing chamber  340  is first under vacuum, and a step in the source gas delivery method is to place open/close valve  324  in the open position to allow saturated precursor vapor to enter pipe  322  by diffusion. Precursor vapor then diffuses through pipe  326  to processing chamber  340 . The processing chamber is filled with precursor material and equilibrium is eventually approximated between precursor chamber  304  and processing chamber  340  over time. Once a selected pressure is reached within processing chamber  340 , open/close valve  324  is closed to stop precursor vapor from entering processing chamber  340 . A pressure sensor  360  is used to determine the overall pressure in the processing chamber. Reactant gases are then delivered to the processing chamber to a target overall pressure, after which the processing chamber is activated to start film formation. 
     In another embodiment, as shown by path  510  in FIG. 5 in conjunction with FIG. 2A, reactant gases are first loaded into processing chamber  340  to a selected pressure. Open/close valve  324  is opened and precursor vapor is diffused into processing chamber  340  to a target overall pressure. Open/close valve  324  is then closed and the processing chamber is activated to start film formation. 
     Generally, no more precursor material is required during processing in the processing chamber once the target overall pressure is achieved. Advantageously, this method prevents precursor material from having to be vented after the processing and thus saves precursor material. Since no carrier gas is used in this embodiment, fluctuations in precursor vapor concentration associated with carrier gas use are eliminated. Further, since no bubbling takes place, the precursor vapor concentration does not fluctuate with changing levels of precursor liquid. Another advantage of the present invention is that any impurities or decomposed products from the precursor liquid are left in the precursor chamber since only the precursor vapor is delivered to the processing chamber. Additionally, no heating elements are necessary to heat the vapor pathway from the precursor chamber to the processing chamber because, unlike vaporizer systems, the temperature difference between the precursor chamber and the vapor pathway is minimal. In addition, the vapor pathway can be made with a large enough diameter to increase the throughput of the precursor vapor while minimizing the possibility of condensation. 
     In an alternative embodiment, as shown by path  520  in FIG. 5 in conjunction with FIG. 2B, flow gas may travel along a first pipe  310  to an open/close valve  312 . An open end  316  of pipe  314  is located in a space  330  above the precursor liquid in precursor chamber  304 . If open/close valve  312  is in the open position, flow gas enters pipe  314  and exits at open end  316  into space  330 . Space  330  is initially under vacuum and only precursor vapor will occupy space  330  as the precursor liquid is heated. Once precursor vapor saturation has been reached, open/close valves  312  and  324  are opened and the flow gas is used to directly propel the saturated precursor vapor to the processing chamber, without bubbling, until a target overall pressure is reached. Inert flow gases, such as Ar, He, O 2 , and N 2 , may be delivered to precursor chamber  304  at flowrates of between approximately 100 cc/min to approximately 10,000 cc/min, to propel the saturated precursor vapor. Further, the flow gas may be metered in this embodiment to determine the partial pressure contribution of the flow gas in order to compensate for an increase in the overall pressure within the processing chamber. Flow gas effects on vapor concentration are negligible, since no bubbling occurs and the flow gas is used to only add kinetic energy to the saturated precursor vapor for delivery to processing chamber  340 . Once the target overall pressure is reached, the processing chamber is activated to start film formation. 
     In another embodiment, as shown by path  530  in FIG. 5 in conjunction with FIG. 2B, reactant gases are first loaded into processing chamber  340  to a selected pressure. Then open/close valves  312  and  324  are opened and inert flow gas is used to propel the saturated precursor vapor to the processing chamber until a target overall pressure is reached, after which the processing chamber is activated to start film formation. 
     In accordance with one embodiment of the present invention, precursor liquid TAETO is heated between approximately 50° C. to approximately 220° C., achieving saturated vapor pressures of between approximately 0.001 Torr (˜0.13 Pa) to approximately 100 Torr (˜13 kPa), respectively. The processing chamber is targeted to achieve an overall pressure ranging between approximately 0.001 Torr (˜0.13 Pa) to approximately 760 Torr (˜1 MPa). The saturated precursor vapor has a mole percentage in the processing chamber ranging from approximately 0.1% to approximately 50%. A layer of tantalum oxide (Ta 2 O 5 ) may be grown to thicknesses ranging between approximately 50 Å to approximately 500 Å. 
     In one example, precursor liquid TAETO is heated to 130° C. to achieve a saturated vapor pressure of 0.2 Torr (˜26 Pa) (FIG.  3 ). The precursor vapor is delivered to the processing chamber to a pressure of 0.2 Torr (˜26 Pa) followed by delivery of reactant gases to an overall target pressure of 1 Torr (˜133 Pa). In an alternative example, reactant gases are delivered to the processing chamber to a pressure of 0.8 Torr (˜106 Pa) followed by the precursor vapor to an overall target pressure of 1 Torr (˜133 Pa). The mixture of 20% mole percentage of precursor vapor is reacted in the processing chamber at 450° C. for 10 minutes to grow about 100 Å of tantalum oxide. 
     In another example, precursor liquid TAETO is heated to 200° C. to achieve a saturated vapor pressure of 20 Torr (˜2.6 kPa). The precursor vapor is delivered to the processing chamber to a pressure of 20 Torr (˜2.6 kPa) followed by delivery of reactant gases to an overall target pressure of 100 Torr (˜13 kPa). Alternatively, reactant gases are delivered to the processing chamber to a pressure of 80 Torr (10.6 kPa) followed by the precursor vapor to an overall target pressure of 100 Torr (˜13 kPa). This mixture of 20% mole percentage of precursor vapor is reacted in the processing chamber at 450° C. for 2 minutes to grow about 100 Å of tantalum oxide. 
     The above-described embodiments of the present invention are merely meant to be illustrative and not limiting. It will thus be obvious to those skilled in the art that various changes and modifications may be made without departing from this invention in its broader aspects. Therefore, the appended claims encompass all such changes and modifications as falling within the true spirit and scope of this invention.