WO2006009872A1

WO2006009872A1 - Direct injection chemical vapor deposition method

Info

Publication number: WO2006009872A1
Application number: PCT/US2005/021533
Authority: WO
Inventors: Michael Benjamin Abrams; Gary Stephen Silverman; Ryan Christopher Smith; Jeffery Lee Stricker
Original assignee: Arkema Inc.
Priority date: 2004-06-22
Filing date: 2005-06-16
Publication date: 2006-01-26

Abstract

The present invention provides a method for vaporizing and transporting precursor molecules to a process chamber for deposition of thin films on a substrate. The methods include CVD solvents that comprise ionic liquids. A desired precursor is dissolved in a selected CVD solvent comprising an ionic liquid. The solvent and precursor solution are injected into a packed vaporizer having a counter-current carrier gas flow. The carrier gas strips the precursor from the ionic liquid and transports the precursor molecules in the vapor phase to a deposition chamber. Conventional deposition processes may be used to deposit the desired thin film on a substrate.

Description

DIRECT INJECTION CHEMICAL VAPOR DEPOSITION METHOD

[0001] This application claims the benefit of U.S. Provisional Application Serial No. 60/582,236, filed June 22, 2004.

[0002] The present invention provides methods for vaporizing and transporting precursor molecules for deposition of thin films on a substrate such as in a process chamber or a continuous process. The methods include CVD solvents that comprise ionic liquids. A desired precursor is dissolved in a selected CVD solvent comprising an ionic liquid. The solvent and precursor solution are injected into a vaporizer where an injected carrier gas strips the precursor from the solvent/precursor solution and transports the precursor molecules in the vapor phase to a deposition chamber. Conventional deposition processes may be used to deposit the desired thin film on a substrate.

Field of Invention

[0003] The present invention concerns methods for vapor deposition, and particularly concerns methods for providing volatile precursor molecules to form a thin film on a substrate via vapor deposition.

Background of the Invention

[0004] Chemical vapor deposition (CVD) is one process for forming a thin film, on a substrate. Suitable substrates include semiconductor wafers, glass, or any other material upon which a thin film may be deposited. CVD involves the formation of a non¬ volatile solid film composed of elemental metals or metal compounds on a substrate by the reaction of vapor phase reactants (precursors) that contain desired components of the film. Standard CVD processes utilize a vaporization apparatus to convert a precursor source to the gas phase for transport to a process chamber or apparatus. The vaporization apparatus is connected to a process (or reactor) chamber wherein a deposition substrate, such as a semiconductor wafer or glass, is located.

[0005] CVD (and other thin film vapor deposition) techniques require delivery of a controlled mass of the precursor in the vapor phase. Precise control over the mass of the precursor delivered to the process chamber is needed to reproducibly form a uniform layer of the desired thin film. In addition, the manner of delivery of the precursor must avoid decomposition of the reactive volatile precursor molecules prior to reaching the substrate surface and must not include unwanted volatized elements or compounds.

[0006] Conventional methods of providing a source of vapor-phase precursor molecules include (1) direct vaporization of neat solid or liquid precursors, (2) direct vaporization of a solvent containing the precursor, and (3) distilling precursor molecules from a solvent by bubbling a carrier gas through a volume of the solvent containing the precursor.

[0007] Bulk sublimation of a solid precursor and transport of the vaporized solid precursor to the process chamber using a carrier gas has been practiced. However, it is difficult to vaporize a solid at a controlled rate such that a constant and reproducible flow of vaporized solid precursor is delivered to the process chamber. Lack of control of the rate of delivery of a vaporized solid precursor is (at least in part) due to a changing surface area of the bulk solid precursor as it is vaporized. In addition, many solid precursors will agglomerate upon repeated heating/cooling cycles. The changing surface area of the solid precursor when it is exposed to sublimation temperatures produces a continuously changing rate of vaporization. This is particularly true for thermally sensitive compounds. The changing rate of vaporization thus results in a continuously changing concentration and non-reproducible flow of vaporized precursor delivered for deposition in the process chamber. As a result, film growth rate and the composition of films deposited using such techniques are not adequately controlled. Further, many potential precursor materials decompose when heated to temperatures required for vaporization. [0008] Liquid precursors may be vaporized directly using a bubbler device. A liquid precursor is heated in a reservoir to a temperature at which there is sufficient vaporization to maintain a particular deposition rate. A stream of carrier gas is directed over the precursor or is bubbled through the liquid precursor in the reservoir. The carrier gas transports vaporized precursor molecules to a process chamber or coater for deposition of a CVD thin film. Alternately, liquid precursors may be injected at a known rate into a vaporization chamber through which a carrier gas stream is simultaneously injected. The carrier gas than transports the vaporized precursor molecules to the process chamber for the deposition of a thin film. However, many desirable precursor molecules, when heated to a temperature sufficient to maintain a particular deposition rate will simply decompose in the bubbler.

[0009] It is also possible to dissolve a liquid or solid precursor in a solvent and vaporize the solution directly via injection of the solution into a vaporization chamber. Many desirable precursors are solids at room temperature. In the vaporization apparatus, the solvent and the precursor are quickly heated to the gas phase and are transported to the process chamber, often via a carrier gas stream. In addition, the solvents commonly used to dissolve such precursors typically result in CVD processes where the solvent molecules are carried along with the precursor. Additionally, such solvent molecules have a tendency to decompose on the substrate. Solvent decomposition products, e.g., carbon, are impurities in the thin film resulting in poor thin film quality.

[0010] As an alternative, liquid or solid precursors may be mixed with or dissolved in a solvent and the solvent solution containing the precursor placed in a bubbler device. The solvent containing the dissolved precursor is then heated in a reservoir. As described above for liquid precursors, a stream of carrier gas is directed over or bubbled through the solvent. The carrier gas transports the volatile precursor molecules from the solvent to a process chamber. Unfortunately, typical solvents that will dissolve CDV precursors are typically organic compounds that possess vapor pressures of greater than about 1 Torr at about room temperature. Accordingly, volatilized solvent molecules are often transported to the process chamber along with the precursor molecules. This problem is exacerbated when temperatures above room temperature are needed to volatilize sufficient precursor molecules and/or to maintain a given deposition rate. As a result, solvent molecules or solvent decomposition products are deposited in the film.

[0011] In addition, prolonged exposure of the precursor to the elevated temperatures often required to impart sufficient volatility in a bubbler may result in precursor decomposition. Such variations make consistent delivery of precursor difficult since precursor volatilization can depend on bubbler or precursor temperature, carrier gas flow, carrier gas bubble diameter, etc. Furthermore, the path from the bubbler to the deposition chamber or coater must be heated sufficiently to prevent precursor condensation. Exposure to such elevated temperatures can result in precursor decomposition.

Summary of the Invention

[0012] The present invention provides an improved direct injection method to deliver chemical vapor deposition (CVD) precursors to a deposition chamber or deposition process. The present invention will be described with respect to a closed deposition chamber as would be used for semiconductor processing. The present invention can also be used with respect to "open" CVD deposition processes such as continuous deposition on glass in a float glass process or other processes that employ CVD. In the method of the present invention, a precursor is dissolved in an ionic solvent and the ionic solvent/precursor solution is injected into a vaporization chamber containing a suitable packing. The vaporization chamber may be heated. A suitable carrier gas is injected into the vaporization chamber, in a counter-current flow, to strip the precursor from the ionic solvent. The carrier gas transports the precursor to a deposition chamber. The stripped ionic solvent can be disposed of or recycled to s suitable storage container. The use of an ionic solvent allows for injection of controlled volumes of precursor into the vaporizer and reduces contamination of the precursor delivered as a result of the low vapor pressure of the ionic fluid.

[0013] The vapor deposition methods of the present invention provide for the vaporization and transport of a controlled mass of precursor molecules in the vapor phase. Due to the use of ionic solvents, which have extremely low or substantially no vapor pressures, used in practicing the vapor deposition methods of the present invention, solvent molecules are not transported to the process chamber along with the vaporized precursors. Further, the range of precursor materials that may be vaporized in the solvent without unwanted decomposition of the solvent or vaporization of the solvent itself is increased. Additionally, the vapor deposition methods of the present invention include use of solvents that may be used with conventional direct injection device technology and that are non-corrosive. Moreover, because the present invention uses solvents that exhibit a wide liquid temperature range (i.e., greater than about 100° C), there is a significant increase in the range of materials that may be deposited.

Brief Description of the Drawings

[0014] FIG. 1 is a schematic of an apparatus for chemical vapor deposition using direct injection. ^r

Detailed Description

[0015] The vapor deposition methods of the present invention include chemical vapor deposition. (CVD) solvents that comprise ionic liquids. Such solvents, in contrast to conventional CVD solvents, possess wide liquid temperature ranges (typically greater than about 100° C.) and exhibit substantially no measurable vapor pressure (i.e., less than about 1 Torr at about room temperature). Further, the present methods include ionic liquid CVD solvents that dissolve a wide variety of precursor materials.

[0016] The methods of the present invention further include ionic liquid CVD solvents that are relatively inert and stable. For example, chloroaluminate ionic liquids are air and water sensitive (i.e., such ionic liquids tend to be unstable in the presence of air or water), but hexafluorophoshate, and tetrafluoroborate ionic liquids are not.

[0017] The methods of the present invention include ionic liquids that are liquids at ambient temperature so that dissolution of the precursor molecules may be accomplished without heating the mixture. As mentioned below, the cation of the ionic liquid CVD solvent may be selected for its effect on the melting point of the ionic liquid as well as its solvating properties.

[0018] Physical characteristics of the ionic liquid CVD solvents of the methods of the present invention may be altered in order to allow dissolution and vaporization of a wide variety of precursors. As known to those of ordinary skill in the art, adjustment may be made to the physical properties of a compound to change one or more particular characteristics of the compound. For example, substituting the cation of an ionic liquid and/or substituting the anion will alter the ionic liquid's physical properties.

[0019] Although the vapor deposition methods of the present invention are primarily discussed with reference to chemical vapor deposition, it should be understood that the vapor deposition methods may be applicable to any thin film deposition technique requiring a source of volatile molecules or precursors. Such techniques may include for example, physical vapor deposition, chemical vapor deposition, metal organic chemical vapor deposition, atmospheric pressure vapor deposition, low pressure chemical vapor deposition, plasma enhanced low pressure vapor deposition, molecular beam epitaxy, and atomic layer epitaxy.

[0020] Likewise, although the vapor deposition methods of the present invention are discussed primarily with reference to semiconductor substrates or semiconductor wafers or glass, it should be understood that the substrate may comprise silicon, gallium arsenide, glass, an insulating material such as sapphire, or any other substrate material upon which thin films may be deposited. [0021] A typical chemical vapor deposition system that can be used to perform the deposition methods of the present invention is shown in FIG. 1. The CVD system includes a process (deposition) chamber 10. Process chamber 10 is shown as a closed chamber, an open deposition system is with the scope of the present invention. As is conventional CVD, the CVD process may be carried out at pressures of from about atmospheric pressure down to about 10^'3 Torr. If the process is not to be performed at approximately atmospheric pressure, pressures from about 1.0 to about 0.1 Torr are preferred. In these cases, a vacuum may be created in chamber 10 using a pump 12 (e.g., a turbo pump) and backing pump 14.

[0022] One or more substrates 16 are positioned in the process chamber 10. A constant nominal temperature is established for the substrate 16, preferably at a temperature of about 0° C to about 800° C, and more preferably at a temperature of about 100° C to about 500° C. Substrate 16 may be heated, for example, by an electrical resistance heater 18 on which substrate 16 is mounted. Other known methods of heating the substrate 16 may be utilized.

[0023] A precursor is dissolved in an ionic liquid solvent (as discussed in detail below). The ionic liquid solvent/precursor solution 40 is injected into a packed vaporizer 42. The vaporizer 42 contains a suitable packing 44. A carrier gas is injected into vaporizer 42 at line 47 and flows upward through packing 44. The precursor/ionic liquid^* solvent mixture is injected into vaporizer 42 at 48 and flows downwardly through vaporizer 42. The carrier gas strips the precursor from the precursor/ionic liquid solvent and a precursor/carrier gas mixture exits vaporizer through line 45. The ionic liquid solvent, free of precursor exits vaporizer 42 at line 50 and may be discarded or recycled.

[0024] The precursor/carrier gas mixture travels to process chamber 10 through line 45 and distributor 46. [0025] Vaporizer 42 may be heated by an appropriate heating element or jacket (not shown) if desired. If more than one precursor/ionic liquid solvent mixture is to be employed, multiple vaporizers or injectors (not shown) can be employed to feed into line 45.

[0026] If the ionic liquid solvent is to be recycled, line 50 can feed a mixing reservoir (not shown), which would replace injector system 52. The mixing reservoir would also include means to add a controlled amount of the precursor to the recycled ionic liquid solvent to form the precursor/ionic liquid solvent solution of the desired precursor concentration prior to injection into vaporizer 42.

[0027] While the present invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims and this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.

Claims

CLAIMSWhat is claimed is:

1. A method of vaporizing reactants for vapor depositions of a thin film on a substrate comprising: dissolving a precursor in an ionic liquid to form a precursor/ionic liquid solution; injecting said precursor/ionic liquid into a vaporizer; injecting a carrier gas into said vaporizer whereby said carrier gas strips said precursor from said precursor/ionic liquid solution forming a carrier gas/precursor stream; feeding said carrier gas/precursor stream to a vapor depositions process.

2. The method of claim 1 wherein said vaporizer is a packed column.

3. The method of claim 2 wherein said packed column employs counter- current flow.

4. The method of claim 1 wherein said vaporizer is heated.

5. The method of claim 1 wherein ionic liquid stripped of precursor is withdrawn from said vaporizer.

6. The method of claim 1 wherein said vapor deposition process comprises deposition in a deposition chamber.

7. The method of claim 1 wherein said vapor deposition process is a continuous deposition process.

8. The method of claim 1 wherein ionic liquid is withdrawn form said vaporizer and recirculated into a precursor solution storage container.