Abstract:
A precursor delivery system introduces a multicomponent solid precursor into an evaporator during chemical vapor deposition. The system uses a container having an open bottom with the precursor being placed into the container along with a plurality of beads that do not react with the precursor. The beads, coated with the precursor, exit the container by being transported within the groove of a roller that is rotatably disposed proximate the open bottom of the container and drop out of the groove and into the evaporator with the beads thereby introducing the precursor into the evaporator. A screen is disposed within the evaporator for capturing and retrieving the beads.

Description:
This application claims the benefit of U.S. provisional patent application No. 60/193,752 filed on Mar. 31, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a delivery system for delivering vapors of solid state precursors into a chemical vapor deposition reactor for thin film deposition. 
     2. Background of the Prior Art 
     Chemical vapor deposition has emerged as an important technique for thin film deposition in contemporary microelectronics and other areas. A large variety of materials can be obtained by this method including pure metal films, semiconductors, and dielectric. Although chemical vapor deposition of semiconductors and metals has been investigated for quite some time, only recently has chemical vapor deposition been studied for the formation of different oxide materials. Research into high temperature super conducting thin films has resulted in the application of chemical vapor deposition methods for the production of different ferroelectric, magnetoresistant, thermoelectric, and other electrical, magnetic, and optical materials. 
     In the past, precursor delivery in chemical vapor deposition did not pose problems. Liquid or even gaseous compounds like SiH 4 , AsH 3  or Al(CH 3 ) 3  could be used in the deposition of a variety of semiconductors and metals. Even solid precursors such as CBr 4 , which is used as a p-type dopant in the growth of GaAs layers, is sufficiently stable to be evaporated from a container having a constant high temperature. 
     However, many new materials require less volatile and frequently less stable precursors for their deposition. For example, BaO is a common component of high temperature superconducting, ferroelectric, and other thin film oxide materials [Ba(dpm) 2 ] (dpm=dipivaloymethane) and its derivatives are the most common compounds used to introduce Ba into the films of such materials. Unfortunately, this complex evaporates only at about 250 degrees Celsius and is prone to oligomerization during heating, which decreases its vapor pressure. 
     Liquid precursors are convenient for chemical vapor deposition as they allow the use of a bubbler for their evaporation at temperatures at and around room temperature. This provides a constant precursor vapor pressure during the deposition. However, for some applications, precursor compounds are not known, while for other applications, the compounds are very difficult to synthesize and frequently tend to be unstable as well as air and moisture sensitive. 
     Therefore, many manufactures use solid precursors for the chemical vapor deposition process. For the deposition of multicomponent materials, several precursors are needed, typically one volatile compound for each component, with each compound being evaporated from a separate container. The use of multiple containers results in a large number of process parameters (temperature and flow of carrier gas for each container) which considerably complicates the control and understanding of the overall deposition process. 
     Therefore, a device is needed that will simplify the overall chemical vapor deposition process by evaporating all precursors together. Many solutions have been proposed. One such method dissolves all of the necessary precursors in a suitable solvent with the solution being introduced into the hot zone of an evaporator and evaporated along with the precursors. A micropump or an ultrasonic nebulizer with an inert carrier gas can be used for delivery of the solution into the evaporator. While this method has produced successful result, it has certain disadvantages. The use of the solvent raises the possibility of film contamination from carbon resulting from the decomposed solvent which contamination is highly unfavorable in electronic applications. Additionally, the use of a micropump or nebulizer increases the costs of the process and raises the possibility of nonreproducability of the properties of the film. 
     Another method evaporates all precursors simultaneously by introducing them as a mixture of solid compounds. Unfortunately, particles of many precursors are apt to stick together requiring a special feeder for the introduction of the precursor powder into the evaporator. One such special feeder achieves a continues precursor supply by providing a capillary having a slit along the slide. A mixture of the precursors fills the capillary which is then slowly moved into the hot zone of the evaporator wherein the mixture evaporates. This feeder proves problematic when used with a large mass of precursors, with precursors which melt before evaporation, such as Ba(dpm) 2 , and with mixtures of precursors having substantially different vapor pressures. Another type of solid precursor feeder uses the flash-evaporation technique wherein a mixture of precursors is introduced into the hot zone in small portions. Each portion of mixture must evaporate completely before the next portion is introduced. While such a device can achieve desirable results, the device tends to be complicated and costly. 
     Therefore, there is a need in the art for a precursor delivery system capable of delivering a powder precursor into an evaporator, which system overcomes the above-stated problems in the art. Such a system, which must be relatively simple in design and construction and be relatively inexpensive to manufacture and operate, must be able to deliver most types and mixtures of powder precursors into the evaporator. The system must minimize the potential for contamination of the film being produced and must allow for a high degree of reproducibility of the properties of the film being produced. 
     SUMMARY OF THE INVENTION 
     The powder precursor delivery system for chemical vapor deposition of the present inventions addresses the aforementioned needs in the art. The system, which is relatively simple in design and construction and is relatively inexpensive to manufacture and operate, is able to deliver most types and mixtures of powder precursors into the evaporator. The system minimizes the potential for contamination of the film being produced and allows for a high degree of reproducibility of the properties of the film being produced. 
     The powder precursor delivery system of the present invention is comprised of a housing having an open first top and an open first bottom. A container having a second open top and a second open bottom is disposed within the housing while a roller having a nonstick outer surface and a circumferential groove, is rotatably disposed within the housing and positioned such that a portion of the outer surface faces the second bottom of the container. A precursor is placed into the container as is a plurality of beads wherein the precursor coats the beads. The beads exit the container by being transported within the groove of the roller as the roller is rotated and eventually dropping out of the groove by the force of gravity. The beads are made from an inert material that does not react with the precursor such as stainless steel. The precursor is placed into the container in either powder or crystal form. A motor is operatively connected to the roller for rotating the roller. An evaporator having a open third top attached to the first bottom of the housing and a third bottom is located below the container. A first inlet tube is attached to the evaporator for introducing a first gas into the evaporator. A first screen is disposed within the evaporator while a second screen is disposed within the evaporator downstream of the first screen. An outlet tube extends between the third bottom of the evaporator and a reactor for fluid flow connecting the evaporator with the reactor. A second inlet tube is attached to the outlet tube for introducing a second gas into the outlet tube. A substrate holder is disposed within the reactor while a thermocouple is attached to the holder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation view of the powder precursor delivery system for chemical vapor deposition of the present invention. 
     FIG. 2 is a front elevation view of the container and roller. 
     FIG. 3 is a side elevation view of FIG.  2 . 
     FIG. 4 is a side elevation view of the powder precursor delivery system for chemical vapor deposition within a chemical vapor deposition system. 
     FIG. 5 is a graph illustrating the condensation rate of Ba(dpm)2 on a cold substrate holder as a function of time. 
     FIG. 6 is a graph illustrating the influence of pressure on the copper deposition rate. 
     FIG. 7 is a graph illustrating the temperature dependence of the cobalt deposition rate for Co(aai) 2 . 
     Similar reference numerals refer to similar parts throughout the several views of the drawings. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, it is seen that the powder precursor delivery system for chemical vapor deposition, generally denoted by reference numeral  10 , is comprised of a housing  12  having an open top and an open bottom to which a cap  14  or lid can be removably attached. A container  16  having an open bottom is disposed within the housing  12 . A roller  18  having a Teflon (or similar nonstick substance) outer surface and a groove  20  and having a shaft  22  passing through the roller  18 , is rotatably mounted underneath the open bottom of the housing  12 . A motor  24  is attached to the housing  12  and the shaft  22  is operatively connected to the motor  24 . The motor  24  is electrically connected to an appropriate power source (not illustrated). 
     The open bottom of the housing  12  is connected, via a tube  26 , to an evaporator  28  of any appropriate design, the evaporator  28  being a stainless steel foil tube member having an inlet tube  30  proximate the evaporator&#39;s top and an outlet tube  32  proximate the evaporator&#39;s bottom, the outlet tube  32  being connected to a reactor  34  of any appropriate design and having a substrate  36  mountable therein on a holder  38  to which a thermocouple  40  is attached, the outlet tube  32  itself having an inlet tube  42 . A generally cone-shaped stainless steel screen  44  is located within the evaporator  28  proximate the bottom. A second stainless steel screen  46  is located within the evaporator  28  downstream of the first screen  44 . 
     In order to use the powder precursor delivery system for chemical vapor deposition  10  of the present invention, a desired precursor is obtained and reduced to either a powder or a small crystal form. Small beads  48 , which are made from any inert material that does not react with the precursor or the precursor vapor (such as 1 mm beads of stainless steel for example), are mixed with the precursor in order to dilute the precursor. The mixture is placed into the container  16  within the housing  12  and the motor  24  is activated. The activation of the motor  24  causes rotation of the shaft  22  which in turn causes rotation of the roller  18 . As the roller  18  is dimensioned and positioned relative to the container  16  so that the precursor-coated beads  48  can escape from the bottom of the container  16  only by way of the groove  20 , the rotation of the roller  18  causes the groove  20  to pick up the beads  48  from within the container  16  and transport them along the groove  20  until the beads  48  drop off of the groove  20  and fall into the evaporator  28  wherein the precursor is fast evaporated from the surface of the beads  48 . The approximate precursor supply rate is about 1 mg per minute although this can be varied by changing the motor  24  rotation speed. The precursor vapor travels into the reactor  34  wherein it is deposited on the substrate  36  in the usual way. The first screen  44  captures the beads  48  from where the beads  48  can be retrieved after the deposition process while any other stray particles within the evaporator  28  are captured by the second screen  46  which can be cleaned or replaced as needed. Desired gas is introduced into the evaporator via its inlet tube  30  in usual fashion while desired gas is introduced into the precursor vapor via the lower inlet tube  42 . 
     In order to test the flash-evaporation feeder of the present invention, several experiments were conducted. The evaporator  28  was heated to a temperature high enough for fast evaporation of all precursors tested. The outlet tube  32  between the evaporator  28  and reactor  34  was heated to a temperature that was 10 degrees Celsius higher than the temperature of the evaporator in order to prevent condensation. Hydrogen (H 2 ) and helium (He) of UHP/ZERO grade were used without further purification, the helium being introduced through the upper inlet tube  30  and the hydrogen being introduced through the lower inlet tube  42 . The flow of each gas was controlled by a mass-flow controller with an accuracy rate of about plus/minus 1 sccm. The substrate holder  38  is heated resistively and the substrates  36  were attached to the holder  38  by a silver paste. The temperature of the substrates  36  was controlled by the thermocouple with an accuracy of about plus/minus 2 degrees Celsius. The pressure within the system was controlled with an accuracy of about plus/minus 1 mbar. 
     The evaporation of [Ba(dpm) 2 ] (dpm=dipivaloylmethane) followed by condensation on a cold substrate holder was tested. [Ba(dpm) 2 ] was provided and the temperature of the evaporator was about 250 degrees Celsius, while the substrate holder was at room temperature. Helium carrier gas flow was set at 50 sccm. A powder of the compound was introduced into the container and was delivered to the evaporator at a steady rate. The amount of the compound that condensed on the cold substrate holder was measured and plotted as a function of time and is illustrated in FIG.  5 . As seen, the condensation rate is stable over the period of 70 minutes. No residue or decomposition products were found on the walls of the evaporator or the surface of the beads. 
     The powder precursor delivery system for chemical vapor deposition  10  was tested for preparations of Cu—Co binary films, such films consisting of ultrafine magnetic cobalt particles embedded in a nonmagnetic copper matrix posses giant magnetoresistance (GMR) properties. In order to deposit these films, depositions of the individual components Cu and Co were studied. 
     Copper films were obtained from Cu(dpm) 2 (dpm=dipivloylmethanate) precursor. The temperature of the evaporator was held at about 150 degrees Celsius, while hydrogen or helium was used as the carrier gas with a flow rate of about 160 sccm. Film deposition was studied in the temperature range of about 300 to about 500 degrees Celsius and a pressure range of about 1 to about 60 mbar. The deposition rate as a function of pressure is illustrated in FIG. 6 wherein it is noted that Cu film cannot be deposited at pressures below about 5 mbar in hydrogen and/or helium flow. A pressure of about 20 mbar can be considered optimal for the deposition of Cu films at about 400 degrees Celsius. 
     Cobalt films were deposited using Co(aai) 2  (aai=acetylacetoniminate) as a precursor. The evaporator  28  was set to a temperature of about 150 degrees Celsius and the hydrogen flow rate was about 160 sccm. Film growth was studied in the temperature range of about 350 to about 500 degrees Celsius. FIG. 7 illustrates the Cobalt deposition rate as a function of temperature. The dependence appears to differ from the classical Arrhenius temperature dependence which could be the result of catalytic influence of the deposition metal Co on the precursor decomposition reaction. Composition of the resulting Co films was studied by X-ray Photoelectron Spectroscopy (XPS). The addition of hydrogen was found to decrease the carbon content from about 15 to about 4 at. %., while the concentration of oxygen in the films remained as high as 13%. Cobalt grain size, according to SEM, was only about 20 nm. 
     Binary films of copper-cobalt heterogeneous alloy were prepared from a mixture of Cu(dpm) 2  and Co(aai) 2  precursors. Powders of the compounds were mixed, beads  48  were added, and the mixture was loaded into the container  16 . Pressure in the system was maintained at about 20 mbar, which is required for the Cu deposition process. The temperature of the evaporator  28  was about 150 degrees Celsius, the hydrogen flow rate was about 160 sccm, and the deposition temperature was about 400 degrees Celsius. The Cu:Co precursor mixture of 3:1 atomic ratio results in films consisting of about 63.7 at. % of Cu and about 36.3 at. % of Co according to XPS. Metal Cu and Co phases were detected in the films by X-ray Diffractory (XRD). Intensity of the Co reflections was low and the peaks were broad, which can be considered as an indication of the small size of the Co grains. Only big grains with a diameter of about 200 nm were seen on SEM microphotographs, and these can be assigned to copper. 
     The Co precursor containing only Co—N bonds was also tested in the last setup. The cobalt salt of 3,5-dimethylpyrozolylborate, Co([3,5-MePz] 2 BH 2 ) 2 , can be sublimed at sufficiently low temperatures (about 180 degrees Celsius) to be used as a precursor in a chemical vapor deposition process. The high thermal stability of the compound does not allow it to be used for the deposition of cobalt films. Even at temperatures of about 600 degrees Celsius, vapors of this compound do not decompose on a dielectric or on a conducting surface. However, a mixture of Cu(dmp) 2  and Co([3,5-MePz] 2 BH 2 ) 2  at 20 mbar give a film containing both Cu and Co. The Cu:Co precursor mixture of 1:4 atomic ratio resulted in a film composition of 36 at. % of Cu and 64 at. % Cu according to XPS. Metal Cu and Co phases are found by XRD in this film. 
     While the invention has been particularly shown and described with reference to an embodiment thereof, it will be appreciated by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention.