Abstract:
A chemical vapor deposition (CVD) apparatus for depositing a low vapor pressure copper precursor onto a silicon wafer. The CVD apparatus includes a CVD reaction chamber with an interior containing a substrate holder adapted to support a substrate, such as a silicon wafer, at a predetermined position within the CVD reaction chamber. An ultrasonic nebulizer is operatively connected to the CVD reaction chamber and is adapted to connect to a source of liquid precursor. The ultrasonic nebulizer has an atomizing discharge end adapted to atomize the liquid precursor and deposit the atomized precursor onto a substrate supported by the substrate holder. A gas distribution ring is disposed within the interior of the CVD reaction chamber for discharging a directionally oriented gas into the atomized precursor to direct the atomized precursor toward the substrate. Additional embodiments and methods for depositing the precursor are described.

Description:
FIELD OF THE INVENTION 
     This invention is directed to method and apparatus for depositing precursor onto a substrate and, more specifically, depositing a low vapor pressure copper precursor onto a silicon wafer using an ultrasonic nebulizer. 
     BACKGROUND OF THE INVENTION 
     In the manufacture of semiconductor wafers and of other similarly manufactured articles, sequences of processes including coating, etching, heat treating and patterning are sequentially employed. Most of these processes involve the chemical or physical addition or removal of material to or from a surface of a substrate, usually transported as a vapor. 
     Certain coating processes in such sequences are preformed by chemical vapor deposition (CVD). CVD is preferred, for example, in applying films to the differently facing surfaces of holes through underlying layers, as, for example, to apply conductive films for the purpose of making interconnections across insulating layers and the like. The quality of the overall semiconductor wafer can be significantly affected by the integrity of the coating deposited during the CVD process. Therefore, great attention should be used during the precursor deposition step of the manufacturing process. 
     One typical apparatus for applying metallic coatings to semiconductor wafers, such as those made from silicon, includes a CVD reaction chamber to which liquid precursor is supplied via a mass flow controller. Prior to entering the CVD reaction chamber, the liquid precursor is fed through an atomizer which atomizes the liquid precursor as it enters the CVD reaction chamber. The atomization of the liquid precursor and subsequent evaporation into a gas phase is an essential step in the deposition process. To rapidly vaporize a low volatility liquid material into a gas phase, it is best to atomize the liquid into micron-size droplets first. This process increases the surface-to-volume ratio of the liquid precursor leading to increased evaporation rates. 
     Traditional atomizers use a carrier gas pressure differential in the vaporization process. This pressure differential can significantly decrease the temperature of the atomizing region because of adiabatic expansion. Consequently, the low temperature can slow down the evaporation process and even freeze the liquid precursor. Freezing precursor is especially problematic when depositing particular types of copper-based precursors onto a silicon wafer. 
     Another disadvantage of traditional atomizers is that they generate course droplets, i.e., greater than 100 microns. Because of their relatively large size, these droplets evaporate slowly with respect to their rate of travel through the vaporizer volume. The relatively large droplet size increases the probability of droplets colliding with each other or with the CVD reaction chamber wall before they evaporate completely. Collisions with the wall of the reaction chamber can lead to copper deposition onto the CVD reaction chamber walls. Collisions between droplets can lead to the combination of two smaller droplets into a single, larger droplet, further increasing the time for complete evaporation. 
     What is needed, therefore, is an apparatus and method for atomizing liquid precursors quickly and efficiently. More specifically, a liquid precursor atomization apparatus should atomize liquid precursor into small droplets to increase their evaporation rate. 
     SUMMARY OF THE INVENTION 
     The problems discussed above are overcome by the present invention which utilizes an ultrasonic nebulizer cooperating with a CVD reaction chamber in which semiconductor wafers, such as silicon wafers, are coated. The ultrasonic nebulizer converts high frequency electrical energy into mechanical displacement to precisely atomize small volumes of the liquid precursor into micron-size droplets. The ultrasonic nebulizer mounts directly onto the,cover member of the CVD reaction chamber, minimizing the distance that the vaporized precursor travels before reaching the silicon wafer onto which the precursor vapor is to be deposited. Minimizing the precursor travel distance also minimizes the opportunity for the precursor to contact other surfaces of the CVD reaction chamber which are not intended to receive the precursor deposition. Additionally, thermal convection from a heated sweeping gas assists in evaporating the precursor droplets atomized by the ultrasonic nebulizer. The heated sweeping gas replaces the conventional method of evaporation which relies upon the droplet impinging upon a heated surface which is at a temperature higher than the vaporization temperature for the liquid precursor. Preferably, the heated sweeping gas has a high heat capacity and low molecular weight such as hydrogen, helium, and argon. Use of these low molecular weight gases yields high binary diffusivity which greatly reduces evaporation time. 
     Therefore in accordance with the principles of the present invention a CVD apparatus for depositing a precursor onto a substrate includes a CVD reaction chamber with an interior containing a substrate holder adapted to support a substrate at a predetermined position within the CVD reaction chamber. An ultrasonic nebulizer is operatively connected to the CVD reaction chamber and is adapted to connect to a source of liquid precursor. The ultrasonic nebulizer has an atomizing discharge end adapted to atomize the liquid precursor and deposit the atomized precursor as a vapor onto a substrate supported by the holder. A gas distribution member is disposed within the interior of the CVD reaction chamber for discharging a directionally oriented gas into the atomized precursor to direct the atomized precursor toward the substrate. In one aspect of this embodiment, the directionally oriented gas is heated to further assist in the vaporization of the atomized precursor. 
     In one embodiment, a vaporization zone is provided around the ultrasonic nebulizer and the gas distribution ring. Specifically, side walls, a vaporizer plate, and the cover member of the CVD reaction chamber define the boundaries of the vaporization zone. The side walls particularly prevent any unvaporized precursor droplets from bypassing the vaporizer plate and possibly contacting the silicon wafer or the side walls of the CVD reaction chamber. Preferably, the vaporizer plate is heated to a temperature above the vaporization temperature of the particular precursor being deposited. 
     In another embodiment, the CVD reaction chamber includes a diffuser associated with the ultrasonic nebulizer and the gas distribution ring. The diffuser serves to maintain the atomized precursor and sweeping gas mixture with a uniform velocity front as it travels toward the silicon wafer. By properly choosing the diffuser geometry, the spread of the vaporized precursor is sufficient to cover the substrate surface without unfavorably contacting the CVD reaction chamber walls. 
     Various additional advantages, objects and features of the invention will become more readily apparent to those of ordinary skill in the art upon consideration of the following detailed description of the presently preferred embodiments taken in conjunction with the accompanying drawings. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic representation of a CVD apparatus utilizing an ultrasonic nebulizer affixed to a CVD reaction chamber; 
     FIG. 1B is another schematic representation of a CVD apparatus utilizing an ultrasonic nebulizer affixed to a CVD reaction chamber; 
     FIG. 2 is a enlarged cross-sectional view of one embodiment of the CVD apparatus of the present invention utilizing an ultrasonic nebulizer affixed to a CVD reaction chamber; 
     FIG. 3 is a enlarged cross-sectional view of another embodiment of the CVD apparatus of the present invention utilizing an ultrasonic nebulizer affixed to a CVD reaction chamber; and 
     FIG. 4 is a enlarged cross-sectional view of still another embodiment of the CVD apparatus of the present invention utilizing an ultrasonic nebulizer affixed to a CVD reaction chamber. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As shown schematically in FIG. 1A, a CVD apparatus  10  integrates an ultrasonic nebulizer  12  with a CVD reaction chamber  14  in accordance with the principles of the present invention in order to overcome the known problems of precursor injection using traditional atomizers. The CVD apparatus  10  is connected to a source of liquid precursor  16  which is propelled by a source of pressurized push gas  18  to a mass flow controller  20 . The mass flow controller  20  precisely measures or meters the flow of the liquid precursor into the ultrasonic nebulizer  12 . One particular mass flow controller suited for use in this invention is Liquid-Flow model number LMA-05-3-E-0 manufactured by Bronkhorst and available through Porter Instruments, Scotts Valley, Calif. 95066. Although the CVD apparatus  10  is capable of depositing a wide variety of precursors onto a semiconductor wafer, such as a silicon wafer, the CVD apparatus is especially adapted to use a copper-based precursor, such as CupraSelect™ Blend also know as “Copper-HVAC-TMVS”, to deposit a copper film onto the silicon wafer. CupraSelect™ Blend is available from Schumacher, Carlsbad, Calif. 92009. A source of sweeping gas  22  is connected to the CVD reaction chamber  14 . Advantageously, the sweeping gas is of the type that does not react with the copper-based precursor, such as for example hydrogen or helium. One advantage of using hydrogen is that it creates a reducing environment which prevents the deposited metal from oxidizing. One advantage of using helium is that it has a high specific heat. An ultrasonic energy generator  24  is connected to the ultrasonic nebulizer  12  to provide ultrasonic energy to the ultrasonic nebulizer  12  sufficient to atomize the liquid precursor. 
     The schematic in FIG. 1B illustrates the liquid precursor being delivered to the reaction chamber  14  by a slightly different technique. A calibrated pump  21  such as the type used for liquid chromatography replaces the mass flow controller  20 . One advantage of using the calibrated pump  21  is that it does not require the use of the pressurized push gas  18 , thereby reducing the complexity of the CVD apparatus  10 . 
     With reference to FIG. 2, a particular embodiment of the CVD apparatus  10  is shown in accordance with the principles of the present invention. The CVD reaction chamber  14  has side and bottom walls  30 ,  32  and a cover member  34  sealingly engaging the side walls  30  to form an enclosed chamber. Preferably, the side walls  30  are cylindrical and sized to withstand a vacuum within the CVD reaction chamber  14 . Protruding through the bottom wall  32  is a support member  36  to which a heater housing  38  is affixed. The heater (not shown) contained within the heater housing  38  is a resistance heater. Further details about the construction and operation of the heater can be found in U.S. Pat. No. 5,562,947, which is incorporated fully herein by reference. A susceptor  40  is operatively disposed atop the heater housing  38 , and a substrate, such as a silicon wafer  42 , rests upon the susceptor  40 . During the deposition process, the heater contained within the heater housing  38  heats up the susceptor  40  and thus the silicon wafer  42 . Preferably, the silicon wafer is maintained at a temperature of about 200° C. At this temperature the susceptor is preferably an aluminum plate. For temperatures substantially greater than 200° C., the susceptor is preferably a nickel alloy plate. 
     The CVD reaction chamber  14  also includes an annular exhaust baffle  48  mounted to the side walls  30 . The exhaust baffle creates an annular space  50  around the susceptor  40  through which exhausted process gasses pass. Generally, the exhaust baffle  48  is concentrically aligned with the susceptor  40  such that the annular space  50  between the exhaust baffle  48  and the susceptor  40  is uniform. However, the exhaust baffle includes a ring member  52  which can be shaped so that the annular space  50  is not uniform around the susceptor  40 , allowing more exhaust gas to pass through a wide point in the annular space  50  than a narrow point. Because the ring member  52  can be specific tailored to create an asymmetric annular space  50 , the flow of process gas over the silicon wafer  42  can be optimally tailored. Additional aspects of the exhaust baffle  40  can be found in U.S. Pat. No. 5,356,476, which is incorporated fully herein by reference. 
     The ultrasonic nebulizer  12  sealingly engages cover member  34  via seal member  58 . The ultrasonic nebulizer  12  includes a liquid precursor feed channel  60  through which the liquid precursor from the precursor source  16  travels. The liquid precursor feed channel  60  extends through the ultrasonic nebulizer  12  and through a throughhole  62  in the cover member  34  and terminates at an atomizing discharge end  64 . The ultrasonic nebulizer  12  further includes a rear horn  66  which encircles the liquid precursor feed channel  60 . The ultrasonic nebulizer  12  also includes a plurality of piezoelectric-transducers  68  which encircle the liquid precursor feed channel  60 . The ultrasonic energy generator  24  is operatively connected to the piezoelectric transducers  68  via electrical connector  70 . 
     In operation, the piezoelectric transducers  68  receive high frequency electrical energy from the ultrasonic energy generator  24 , and convert that energy into vibratory mechanical motion at the same frequency. The piezoelectric transducers  68  are operatively coupled to the rear horn  66  which amplifies the motion of the piezoelectric transducers  68 . The excitation created by the piezoelectric transducers  68  produces standing waves along the length of the liquid precursor feed channel  60 , the amplitude of which is maximized at the atomizing discharge end  64 . This exaggerated excitation at the atomizing discharge end  64  atomizes the precursor as it is discharged therefrom. One particular ultrasonic nebulizer  12  suited for use in the CVD apparatus  10  is Model 12335 available from Sono-Tek, Milton, N.Y. 12547. 
     A gas distribution ring  78  is disposed beneath and coaxially aligned with the longitudinal axis of the liquid precursor feed channel  60 . A gas supply tube  80  is connected to the gas distribution ring  78  to which it supplies sweeping gas provided by the sweeping gas source  22 . The gas distribution ring  78  includes a plurality of evenly spaced apart discharge holes  82  pointed inwardly toward the center of the gas distribution ring  78 . Preferably the gas distribution ring  78  is made from ¼ inch OD stainless steel tube. The discharge holes  82  are about 0.5 mm in diameter and spaced about 1 cm apart. The gas supply tube  80  is also made from ¼ inch OD stainless steel tube. The sweeping gas has a flow rate in the range of about 50-500 sccm, preferably 100 sccm. The sweeping gas can be distributed through the distribution ring  78  at pressures in the range of about 0.5 to about 5.0 Torr and temperatures in the range of about 0 to 100 C. 
     The sweeping gas discharged from the discharge holes  82  mixes with the atomized precursor, giving the overall gas mixture a substantial velocity in the direction of the silicon wafer  42 . Depending on the particular deposition process, the sweeping gas is discharge at about 20 C. At this temperature the sweeping gas contributes little to the evaporation of the precursor exiting the ultrasonic nebulizer  12 , but instead serves primarily to direct the precursor towards the silicon wafer  42 . However, it may be advantageous to heat the sweeping gas to assist in the evaporation process. In this instance, the sweeping gas is heated to between about 50-100 C., preferably about 60 C. As the heated sweeping gas mixes with the atomized precursor discharged from the atomizing discharge end  64  of the ultrasonic nebulizer  12  the vaporization rate of the atomized precursor increases noticeably. The waste exhaust gases exit from the otherwise sealed CVD reaction chamber  14  via exhaust port  84 . It will be appreciated that the gas distribution ring  78  could be removed altogether from the CVD reaction chamber  14 . However, the vaporization effectiveness would be significantly decreased, possibly leading to unvaporized precursor contacting the side or bottom walls  30 ,  32  or the silicon wafer  42 . 
     In operation, liquid precursor from source  16  is supplied to the liquid precursor feed channel  60  on the ultrasonic nebulizer  12 . The liquid precursor, such as CupraSelect™ Blend, enters the ultrasonic nebulizer  12  at a flow rate in the range of about 0.2 to 3.0 ml/min, preferably at about 0.5 ml/min and a temperature in the range of about 0-60 C., preferably about 20 C. Increasing the flow rate has the effect of increasing the copper film deposition rate. Advantageously, any pressure which allows the precursor to flow through the tubing is workable, preferably the pressure is between about −5 to 5 psig, and most preferably atmospheric pressure. The ultrasonic energy generator  24  energizes the piezoelectric transducers  70  which establishes a standing wave along the liquid precursor feed channel  60  and atomizes the liquid precursor traveling therethrough. The atomized precursor discharges from the atomizing discharge end  64  and then mixes with the heated sweeping gas discharged from the discharge holes  82  in the gas distribution ring  78 . The atomized precursor vaporizes as it is directed toward the silicon wafer  42 . This entire deposition process is conducted while the CVD reaction chamber  14  is held at a pressure in the range of about 0.1 to 5 Torr, and preferably 0.5 Torr. After the silicon wafer  42  is coated, the supply of liquid precursor to the ultrasonic nebulizer  12  is stopped, the coated silicon wafer  42  is removed, and another uncoated silicon wafer  42  is placed atop the susceptor  40 . 
     Another embodiment similar to embodiment described above is shown in FIG.  3 . To further ensure that all of the atomized precursor vaporizes before reaching the silicon wafer or the side and bottom walls  30 ,  32 , a vaporization zone  90  is provided around the ultrasonic nebulizer  12  and the gas distribution ring  78 . Specifically, side walls  92 , vaporizer plate  94 , and cover member  34  define the boundaries of the vaporization zone  90 . The side walls  92  particularly prevent any unvaporized precursor droplets from bypassing the vaporizer plate  90  and possibly contacting the silicon wafer  42  or the side walls  30  of the CVD reaction chamber  14 . 
     The vaporizer plate  94  has a plurality of throughholes  96  oriented perpendicularly to the surface of the silicon wafer  42 . Advantageously, the vaporizer plate  94  is made from thermally conducting metal which is corrosion resistant, such as a commercially pure nickel, a nickel-base alloy such as Hastalloy C-22, or an anodized aluminum. Preferably, the vaporizer plate  94  is heated to a temperature above the vaporization temperature of the particular precursor being deposited. For example, if CupraSelect™ Blend, a copper-based precursor, is utilized, the vaporizer plate  94  is heated to a temperature in the range of about 60-100° C. Consequently, any liquid droplets of precursor that do contact the vaporizer plate  94  will be vaporized instead of passing onto the silicon wafer  42 . The throughholes  96  in the vaporizer plate  94  allow the sweeping gas and the precursor vapor to pass therethrough in a direction substantially toward the surface of the silicon wafer  42 . 
     Another embodiment similar to that shown in FIG. 2 is illustrated in FIG.  4 . In this embodiment, a diffuser  102  is mounted to the cover member  34 . The diffuser  102  is shaped substantially as a truncated cone with side walls  104  extending downwardly from the cover member  34  and diverging outwardly toward the side walls  32  of CVD reaction chamber  14 . The profile of the side walls  104  is shaped so that the mixture of atomizing precursor and sweeping gas maintain a uniform velocity profile as the mixture flows toward the silicon wafer  42 . That is, the diffuser  102  controls the expansion of the gaseous mixture as it travels toward the silicon wafer  42  such that the velocity of the gaseous mixture near the side walls  104  is approximately equal to the velocity along the diffuser&#39;s center line. By maintaining a uniform flow velocity, the vaporized precursor more uniformly coats the surface of the silicon wafer  42 . 
     While the present invention has been illustrated by a description of various preferred embodiments and while these embodiments have been described in considerable detail in order to describe the best mode of practicing the invention, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications within the spirit and scope of the invention will readily appear to those skilled in the art. The invention itself should only be defined by the appended claims, wherein we claim: