JP4595356B2 - Raw material vaporizer for metalorganic chemical vapor deposition equipment - Google Patents

Description

  The present invention relates to a raw material vaporizer for a metal organic chemical vapor deposition apparatus capable of liquid-filled equilibrium vaporization, and a vaporizer capable of continuous supply of raw materials and stable vaporization during film formation by the apparatus About.

  A raw material gas obtained by vaporizing various organometallic compounds using a vaporizer is conveyed into a reaction chamber using a carrier gas for decomposition, and the decomposition product is deposited on a substrate disposed in the reaction chamber. The film forming method, that is, the metal organic chemical vapor deposition (MOCVD) method, is (i) excellent in step coverage, (ii) a dense film can be obtained, and (iii) homogeneous formation on a large area wafer. Since it has advantages such as being capable of forming a film and (iv) being capable of lowering the process temperature, it is widely used in the field as a film forming method suitable for high integration and mass production.

  The raw material vaporization method in this metal organic chemical vapor deposition method includes a simple solid sublimation method in which a sublimation gas obtained by heating a solid raw material contained in a container is transferred into a reaction chamber using a carrier gas, and a powder raw material. A supported solid sublimation method that suppresses the change in the amount of vaporization accompanying a change in surface area by supporting it on a porous ceramic ball, a liquid bubbling method that transports a source gas corresponding to vapor pressure through a carrier gas into a heated liquid source, and the source in a liquid state A liquid supply method for supplying the gas into the vaporizer is known.

  As a more specific simple solid sublimation method, (i) once the organic alkaline earth metal complex is melted, the melt is cooled and solidified in a raw material container, and then this is less than the melting point of the complex. Vaporize at a temperature, or cast the melt into a cylindrical container so that the thickness in the longitudinal axis direction is substantially constant, and then pass the carrier gas in the longitudinal direction to allow the complex to have a melting point or lower. A method of vaporizing at a temperature (see Patent Document 1), and (ii) a method of heating and vaporizing bis (cyclopentadienyl) ruthenium having an average particle diameter of 0.03 to 5.0 mm while flowing a carrier gas (Patent) Reference 2) is exemplified.

  However, the simple solid sublimation method has a problem that the vaporization amount is not stable due to the change of the raw material surface area accompanying sublimation, and it is not suitable for industrialization because the raw material cannot be continuously filled. The sublimation method is also unsuitable for industrialization because continuous filling of raw materials is difficult.

  Furthermore, the bubbling method not only has a problem that the entire raw material is deteriorated because it is exposed to heat for a long time, but is also toxic in the film formation using lead zirconate titanate (PZT) or the like as a raw material. Therefore, there is a problem that industrialization is difficult because a high Pb raw material (for example, tetraethyl lead, tetraneopentoxy lead, etc.) must be used.

On the other hand, the liquid supply method is expected to be applied to industrialization in order to transport the raw material in a liquid state. In this method, (i) the raw material liquid is directly fed into a vaporizer heated to a high temperature. Depending on the vaporization conditions for injection, the raw material may be decomposed, and particles are generated because the raw material is transported in a mist state. (Ii) A large amount of solvent gas is transported to the substrate together with the raw material. Therefore, not only carbon is taken into the film as an impurity, but also the burden on the exhaust system is increased and maintenance costs are increased. (Iii) Tetrahydrofuran (THF), which is frequently used as a solvent, is highly erodible and therefore viton. Fatal damage to O-rings made, increase maintenance costs, and (iv) the use of expensive raw materials is generally low due to the large carrier gas flow rate, resulting in low product costs There is a problem of increasing.
Japanese Patent Laid-Open No. 11-193462 JP 2003-160865 A

  The present invention has the above problems of the liquid supply method in the metal organic chemical vapor deposition method, in particular, the surface area of the raw material changes with the sublimation of the solid raw material, and the temperature of the raw material changes. The problem that a thin film cannot be formed, the thin film forming apparatus must be stopped in order to add a solid raw material, the problem that the operating rate of the apparatus is reduced, and the case of producing a multi-element thin film Provides a raw material vaporizer for metal organic chemical vapor deposition equipment that enables continuous filling and stable vaporization of raw materials by eliminating the problem of fluctuations in the composition of the thin film formed by changes in the liquid flow rate over time It was made to do.

  According to the first aspect of the present invention, “in the raw material vaporizer for a metal organic chemical vapor deposition apparatus, the vapor pressure of the solid raw material present in the vaporizer is kept substantially constant. There is provided a vaporizer comprising an orifice having a pore size.

  According to a second aspect of the present invention, “a raw material vaporizer for a metal organic chemical vapor deposition apparatus, (i) a filling port for filling a liquid raw material, (ii) a solvent contained in the liquid raw material. And (iii) a heater for heating that has a hole diameter large enough to keep the vapor pressure of the solid material in the vaporizer substantially constant when the solid material remaining after the vaporization is removed. The carburetor is provided.

  According to a third aspect of the present invention, “in the method for supplying a raw material gas for a metal organic chemical vapor deposition apparatus, an orifice disposed in a vaporizer, wherein the raw material present in the vaporizer Provided is a method for supplying a source gas, characterized in that the source gas is supplied to an organometallic chemical vapor deposition system through an orifice having a pore size such that the vapor pressure is kept substantially constant. Is done.

According to a fourth aspect of the present invention, “a method of supplying a source gas for a metal organic chemical vapor deposition apparatus having the following steps (i) to (v):
(I) preparing a liquid raw material from a solid raw material using a solvent;
(Ii) introducing the liquid raw material into a vaporizer having an orifice having a pore size such that the vapor pressure of the solid raw material is maintained substantially constant through a pipe connected to the vaporizer; ,
(Iii) evaporating and removing the solvent contained in the liquid raw material in the vaporizer from the vaporizer;
(Iv) generating the source gas in the vaporizer by heating and sublimating the solid source remaining in the vaporizer; and (v) a metal organic chemical vapor deposition apparatus system through the orifice. Is provided.

When the vaporizer according to the present invention is used, since the vapor pressure of the raw material in the vaporizer is kept substantially constant, the vaporization amount of the raw material due to the change in the raw material surface area accompanying sublimation of the raw material is stable. In addition, since the solid raw material can be filled into the vaporizer through the pipe after being prepared in a liquid state using a solvent, it is possible to continuously supply the raw material important for industrial application. Become. Further, when the vaporizer is used, it is sufficient that a predetermined amount or more of liquid raw material is introduced into the vaporizer, and the vaporization amount of the raw material can be appropriately adjusted according to the temperature in the vaporizer. This makes it possible to accurately and stably adjust the composition when creating a multi-element thin film, and there is no problem of composition fluctuations due to changes in liquid flow over time when using a conventional liquid flow controller. .
Therefore, for example, conventionally, since the range of raw material selection is narrow, it is necessary to use highly toxic tetraethyl lead, tetraneopentoxy lead, etc., and therefore, by an organic metal chemical vapor deposition method on an industrial scale. According to the present invention, a lead zirconate titanate thin film that has been difficult to manufacture can also be industrially manufactured by the method.

Hereinafter, the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a schematic configuration diagram showing an embodiment of a metal organic chemical vapor deposition apparatus equipped with a vaporizer according to the present invention.
FIG. 2 is a schematic configuration diagram showing an operation procedure of the vaporizer according to the present invention shown in FIG.

According to the first aspect of the present invention, “in the raw material vaporizer 9 for a metal organic chemical vapor deposition apparatus, the vapor pressure of the solid raw material 10 existing in the vaporizer 9 is maintained substantially constant. The carburetor 9 "is provided, characterized in that it comprises an orifice 8 having a large pore size.
In this case, “the vapor pressure of the solid raw material 10 existing in the vaporizer 9 is kept substantially constant” means that the vapor pressure PmmHg of the solid raw material 10 defined by the set temperature of the vaporizer 9 and the vaporizer 9 The difference from the vapor pressure P′mmHg of the solid raw material 10 measured in the above is maintained in the range of about 0 to 0.5 mmHg, preferably about 0 to 0.2 mmHg, particularly preferably about 0 to 0.05 mmHg. Means that.

In order to satisfy the above-mentioned conditions concerning the vapor pressure of the solid raw material 10, the hole diameter of the orifice 8 disposed in the vaporizer 9 is about 50 to 5000 μm, preferably about 100 to 3000 μm, particularly preferably about 500 to 1500 μm. Set. When the hole diameter of the orifice 8 is smaller than about 50 μm, the film forming speed of the thin film deposited on the substrate 2 becomes practically insufficient, and when the hole diameter of the orifice 8 is larger than about 5000 μm, the inside of the vaporizer 9 The vapor pressure of the solid raw material 10 cannot be kept constant.
A plurality of orifices may be provided in consideration of the volume of the vaporizer 9, the hole diameter of the orifice 8, the film forming speed, the vapor pressure of the solid raw material 10, and the like.

  When the above-described vaporizer 9 according to the present invention is used, the reaction chamber 3 is passed through the orifice 8 provided in the vaporizer 9 as compared with the amount of the raw material gas generated by vaporization inside the vaporizer 9. Since the amount of the raw material gas supplied to is almost negligible, the inside of the vaporizer 9 is always kept in an equilibrium state, and the vaporization amount of the solid raw material 10 is not affected by the surface area of the raw material. Since it is almost determined by the vapor pressure of the raw material, the supply of the raw material gas to the reaction chamber 3 is stably performed for a long time.

  According to the second aspect of the present invention, “a raw material vaporizer 9 for a metal organic chemical vapor deposition apparatus, comprising (i) a filling port 14 for filling a liquid raw material 13, (ii) the liquid raw material 13. An orifice 8 having a pore size such that the vapor pressure of the solid raw material 10 in the vaporizer 9 is kept substantially constant when the solid raw material 10 remaining after removing the solvent contained in the vaporizer 9 is vaporized, and (Iii) The vaporizer 9 "provided with the heater 7 for heating is provided.

The filling port 14 provided in the vaporizer 9 is connected to a pipe 11 communicating with the liquid source source, and the liquid source 13 is filled into the vaporizer 9 through the filling port 14. The diameter of the filling port 14 is not particularly limited, but in the case of a commonly used vaporizer 9 (volume: about 10 to 100 cc), it is usually about 1 to 8 mm, preferably about 2 to 6 mm, particularly preferably about 3-4 mm.
If desired, a plurality of filling ports may be arranged according to the volume of the vaporizer 9 to be used, the size of the diameter of the filling port 14, the continuous use time of the vaporizer 9, and the like.

  The liquid raw material 13 introduced into the vaporizer 9 through the filling port 14 is subjected to a heat treatment by the heating heater 7 disposed at the bottom of the vaporizer 9, thereby the solvent contained in the liquid raw material 13. Is removed by evaporation, and the solid raw material 10 remains at the bottom of the vaporizer 9. The remaining solid raw material 10 is heated to an appropriate temperature using the heater 7 and vaporized to generate a raw material gas inside the vaporizer 9.

  The vaporizer 9 is provided with an orifice 8 having such a diameter that the vapor pressure of the solid raw material in the vaporizer is kept substantially constant when the residual solid raw material 10 is vaporized. The Regarding the form of the orifice 8, the above description of the vaporizer according to the first aspect of the present invention is applied as it is.

Therefore, (i) a filling port 14 for filling the liquid raw material 13 and (ii) the solid raw material in the vaporizer 9 upon vaporization of the solid raw material 10 remaining after removing the solvent contained in the liquid raw material 13 A raw material vaporizer 9 for a metal organic chemical vapor deposition apparatus according to the present invention, comprising an orifice 8 having a pore size such that the vapor pressure of 10 is kept substantially constant, and (iii) a heater 7 for heating. When used, the amount of raw material gas supplied to the reaction chamber 3 through the orifice 8 provided in the vaporizer 9 is compared with the amount of raw material gas generated by vaporization inside the vaporizer 9. Since it is almost negligible, the inside of the vaporizer 9 is kept in an ordinary equilibrium state, and the vaporization amount of the solid raw material 10 is almost determined by the vapor pressure of the raw material without being influenced by the surface area of the raw material. To the reaction chamber 3 The supply of material gas is stably performed for a long time.

  According to the third aspect of the present invention, “in the method of supplying a source gas for a metal organic chemical vapor deposition apparatus, the orifice 8 is disposed in the vaporizer 9 and exists in the vaporizer 9. Supply of the source gas, characterized in that the source gas is supplied to the organometallic chemical vapor deposition system through an orifice 8 having a pore size such that the vapor pressure of the source is kept substantially constant. Method "is provided.

  The metal organic chemical vapor deposition apparatus of the embodiment shown in FIG. 1 includes (i) a reaction chamber 3 that accommodates a substrate 2 and a substrate heater 1, (ii) an exhaust system that includes a vacuum pump 5 ', and (iii) a carrier gas. A source of argon gas, a carrier gas supply system including a flow rate regulator 16 and a preheater 4, and (iv) a source of oxygen gas as a reaction gas, a reaction gas including a flow rate regulator 16 ′ and a preheater 4 ′ Consists of supply system. In this case, the carrier gas, the reaction gas, and the raw material gas before reaching the substrate 2 are kept warm by the heat retaining heater 12 attached to the lower region of the reaction chamber 3.

  A vaporizer 9 is connected to a lower region of the reaction chamber 3 to which the heat retaining heater 12 is attached, and the lower region and the vaporizer are communicated with each other through an orifice 8 provided in the vaporizer. The orifice 8 has a pore size such that the vapor pressure of the solid raw material present in the vaporizer is kept substantially constant. Regarding the form of the orifice 8, the above description of the vaporizer according to the first aspect of the present invention is applied as it is.

  Therefore, in the method of supplying a source gas for a metal organic chemical vapor deposition apparatus, the vapor pressure of the raw material existing in the vaporizer 9 at the orifice 8 disposed in the vaporizer 9 is substantially constant. According to the method of the present invention in which the raw material gas is supplied to the organometallic chemical vapor deposition apparatus system through the orifice 8 having such a large pore diameter, the raw material gas generated by vaporizing inside the vaporizer 9 can be obtained. Compared with the amount, the amount of the raw material gas supplied to the reaction chamber 3 through the orifice 8 provided in the vaporizer 9 is almost negligible, so the inside of the vaporizer 9 is always kept in an equilibrium state. In addition, since the vaporization amount of the solid raw material 10 is substantially determined by the vapor pressure of the raw material without being influenced by the surface area of the raw material, the supply of the raw material gas to the reaction chamber 3 is performed stably for a long time. It is.

According to a fourth aspect of the present invention, “a method of supplying a source gas for a metal organic chemical vapor deposition apparatus having the following steps (i) to (v):
(I) preparing a liquid material 13 from a solid material using a solvent;
(Ii) The liquid raw material 13 was connected to the vaporizer 9 into a vaporizer 9 having an orifice 8 having a pore size such that the vapor pressure of the solid raw material was maintained substantially constant. Introduced through pipe 11,
(Iii) evaporating and removing the solvent contained in the liquid raw material 13 in the vaporizer 9 from the vaporizer 9;
(Iv) The raw material gas remaining in the vaporizer 9 is heated and sublimated to generate the raw material gas in the vaporizer, and then (v) the raw material gas is passed through the orifice 8 to the organometallic chemical vapor phase. To the deposition system is provided.

The liquid raw material 13 is obtained by dissolving, dispersing, suspending, or emulsifying the solid raw material 10 as a thin film raw material deposited on the substrate 2 installed in the reaction chamber 3 in a solvent. The liquid raw material 13 is most preferably a solution.
The concentration of the liquid raw material 13 may be appropriately selected according to the type of the solid raw material 10, the type of the solvent, and the charging efficiency of the raw material, and is not particularly limited, but is usually about 0.1 to 5 mol / l. , Preferably about 0.2 to 2 mol / l, particularly preferably about 0.5 to 1 mol / l.

The solvent used for preparing the liquid raw material 13 from the solid raw material may be appropriately selected in consideration of the solubility of the solid raw material and the cost and toxicity of the solvent, and is not particularly limited, but the following solvents are exemplified. Tetrahydrofuran, butyl acetate, ethylcyclohexane, toluene, xylene and tetraethylheptanedione.
If desired, two or more of these solvents may be used in combination as appropriate.

  The liquid raw material 13 prepared as described above is supplied into the vaporizer 9 having an orifice 8 having a pore size such that the vapor pressure of the solid raw material 10 is maintained substantially constant. Then, the solvent is introduced through the pipe 11 connected to, and then the solvent contained in the liquid raw material 13 is evaporated and removed from the vaporizer 9.

The operation procedure in this case will be further described with reference to FIG.
In the state shown in FIG. 2A, the valve 6 communicating with the liquid material introduction pipe 11 is opened, and the valve 6 ′ communicating with the pipe 11 and the vacuum pump 5 is closed.
In the state shown in FIG. 2B, the liquid raw material 13 outside the system is filled into the vaporizer from the raw material filling port 14 of the vaporizer 9 via the liquid flow rate regulator 15, the valve 6 and the pipe 11. The
In the state shown in FIG. 2C, after the valve 6 is closed and the valve 6 ′ is opened, the liquid raw material 13 stored at the bottom of the vaporizer 9 is heated by the heater 7, so that the solvent is removed. While evaporating, the inside of the vaporizer 9 is exhausted by driving the vacuum pump 5. By this exhaust, the solvent in the vaporizer is removed from the liquid raw material filling port 14 via the pipe 11 and the valve 6 ′, and the solid raw material 10 remains at the bottom of the vaporizer 9. After removing the solvent, the valve 6 'is closed.

  In the above solvent removal step, the pressure inside the reaction chamber (film formation chamber) 3 is set higher than the pressure inside the vaporizer 9 so that the solvent gas enters the reaction chamber 3 from the vaporizer 9. Can be suppressed.

  Also in the method for supplying a source gas for a metal organic chemical vapor deposition apparatus according to the fourth aspect of the present invention, the above description in the case of the vaporizer according to the first aspect of the present invention is applied as it is with respect to the form of the orifice 8. Is done.

  The raw material gas is generated in the vaporizer by heating and sublimating the solid raw material 10 remaining in the vaporizer 9 by removing the solvent from the liquid raw material 13 in the vaporizer 9 as described above. The source gas is supplied to the metal organic chemical vapor deposition system through the orifice 8.

In the metal organic chemical vapor deposition system, film forming conditions are set as follows, for example. That is, the temperature of the substrate 2 is set to a predetermined temperature (for example, 500 ° C.) using the substrate heater 1, and the pressure inside the reaction chamber 3 is set to a predetermined pressure (for example, 275 Pa) using the vacuum pump 5 ′. The flow rate of argon as the carrier gas of the source gas generated by sublimation of the solid source 10 is adjusted to a predetermined flow rate (for example, 14 sccc / m) using the flow rate regulator 16 and oxygen as the reaction gas The flow rate of the gas is adjusted to a predetermined flow rate (for example, 100 scc / m) using the flow rate adjuster 16 ′, and the temperature inside the vaporizer 9 is set to a predetermined temperature (for example, 150 ° C. using the heater 7). 155 ° C or 160 ° C).
Under such film forming conditions, a desired thin film (for example, a TiO ₂ thin film) can be formed on the substrate 2.

  According to the above-described method for supplying a raw material gas for a metal organic chemical vapor deposition apparatus according to the present invention, the material gas disposed in the vaporizer 9 is compared with the amount of the raw material gas generated by vaporization inside the vaporizer 9. Since the amount of the raw material gas supplied to the reaction chamber 3 through the orifice 8 is almost negligible, the inside of the vaporizer 9 is always kept in an equilibrium state, and the vaporization amount of the solid raw material 10 is the surface area of the raw material. Therefore, the supply of the raw material gas to the reaction chamber 3 is performed stably for a long time.

  Also, according to this raw material gas supply method, after the solid raw material is converted into a liquid state using a solvent, the vaporizer 9 is continuously opened into the vaporizer 9 through the piping 11 from outside the system without opening the vaporizer 9 or Introduced sequentially and then, prior to film formation, only the solvent in the liquid source 13 is evaporated and removed, so that a source gas is always generated from a fresh solid source in the vaporizer 9, and the source gas is supplied to the orifice 8. Through the metal-organic chemical vapor deposition system.

A TiO ₂ thin film was prepared using the metal organic chemical vapor deposition apparatus shown in FIG.
A raw material solution was prepared by dissolving titanium isopropoxypivaloylmethane (2.66 g) in tetrahydrofuran (10 ml) (raw material concentration: 0.5 mol / l). A silicon wafer was used as the substrate 2 for depositing the TiO ₂ thin film.
This raw material solution was introduced into the vaporizer 9 through the pipe 11. Tetrahydrofuran was evaporated and removed from the vaporizer by heating the raw material solution at 80 ° C. for 15 minutes using the heater 7 while evacuating the vaporizer using the vacuum pump 5.
The operation procedure in this case was performed in accordance with the procedure described above with reference to FIG.
Next, the temperature of the substrate 2 is set to 500 ° C. using the substrate heater 1, the pressure inside the reaction chamber 3 is adjusted to 275 Pa using the vacuum pump 5 ′, and the raw material gas generated by sublimation of the solid raw material 10 The flow rate of argon as a carrier gas is adjusted to 14 scc / m using the flow rate regulator 16, the flow rate of oxygen gas as the reaction gas is adjusted to 100 scc / m using the flow rate regulator 16 ′, The temperature inside the vaporizer 9 was set to 150 ° C., 155 ° C. or 160 ° C. using the heater 7 for heating.

Under the above film forming conditions, a TiO ₂ thin film was formed on the substrate, and the film thickness of the thin film was measured over time (after 20 minutes, 25 minutes, 30 minutes and 35 minutes). The measurement results are shown in FIG.
As can be seen from FIG. 3, at any vaporization temperature, the thickness of the titanium dioxide thin film increases almost linearly with increasing deposition time, which uses the vaporizer according to the present invention. In some cases, it indicates that a stable vaporization of the solid source and a constant deposition rate are achieved. FIG. 3 also shows that the deposition rate of the source gas on the substrate can be controlled by appropriately selecting the temperature of the vaporizer 9.

  In industrial production, the operating rate of the production process is a very important requirement from the viewpoint of product cost and the like. When the raw material vaporizer for metal organic chemical vapor deposition apparatus according to the present invention is used, not only the stable vaporization of the raw material important for producing a uniform thin film is guaranteed, but also the operating rate of the apparatus is increased. It is possible to continuously or sequentially supply the important raw materials. Therefore, by incorporating the vaporizer according to the present invention into the mass production process of the thin film by the metal organic chemical vapor deposition method, the operation rate of the mass production process is significantly increased, and this is combined with the stable vaporization of the raw material. Thus, a uniform thin film product can be provided at a low cost.

The raw material vaporizer for metal organic chemical vapor deposition apparatus according to the present invention can be generally applied in the production of thin films by the metal organic chemical vapor deposition method. Among them, it is useful for forming Pb (Zr, Ti) O ₃ , SrBi ₂ Ta ₂ O ₉ , (Bi, La) ₄ Ti ₃ O _{12 and the} like for ferroelectric thin film memory FeRAM.

FIG. 1 is a schematic configuration diagram showing an embodiment of a metal organic chemical vapor deposition apparatus equipped with a vaporizer according to the present invention. FIG. 2 is a schematic configuration diagram showing an operation procedure of the vaporizer according to the present invention shown in FIG. FIG. 3 is a graph showing the relationship between the film thickness of the TiO ₂ thin film on the substrate and the deposition time of the source gas measured in the example of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate heater 2 Substrate 3 Reaction chamber 4 Preheater 4 'Preheater 5 Vacuum pump 5' Vacuum pump 6 Valve 6 'Valve 7 Heating heater 8 Orifice 9 Vaporizer 10 Solid raw material 11 Liquid raw material introduction pipe 12 Heating heater 13 Liquid material 14 Filling port for liquid material 15 Liquid flow rate regulator 16 Flow rate regulator 16 'Flow rate regulator

Claims (5)

A method for supplying a source gas for a metal organic chemical vapor deposition apparatus having the following steps (i) to (v):
(I) preparing a liquid raw material from a solid raw material using a solvent;
(Ii) introducing the liquid raw material into a vaporizer having an orifice having a pore size such that the vapor pressure of the solid raw material is maintained substantially constant through a pipe connected to the vaporizer; ,
(Iii) evaporating and removing the solvent contained in the liquid raw material in the vaporizer from the vaporizer;
(Iv) generating the source gas in the vaporizer by heating and sublimating the solid source remaining in the vaporizer; and (v) a metal organic chemical vapor deposition apparatus system through the orifice. To supply. The method according to claim 1 , wherein the orifice has a pore diameter of 50 to 5000 μm. The method according to claim 1 or 2 , wherein the deposited thin film is lead zirconate titanate. The method according to claim 1 , wherein the liquid raw material is prepared by dissolving the solid raw material in a solvent. Solvent tetrahydrofuran, butyl acetate, ethylcyclohexane, method of claim 1, wherein a solvent selected from toluene, the group consisting of xylene and tetraethyl heptanedione.

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