Crucible material selection for OLED evaporation

Crucible material selection for OLED evaporation

In the manufacturing process of OLED, "evaporation" is the essence part but also the difficulty. It refers to a coating technology in which the materials to be evaporated are heated by resistance or electron beam, and the atoms are vaporized by crucible in the vacuum chamber to reach and adhere to the substrate surface under high vacuum condition.  

In OLED, in addition to luminous materials, even the metal electrode is steamed up in this way, so the quality of the crucible directly affects the yield of THE OLED process.  If the crucible is not suitable, it will cause incomplete evaporation of the material in the crucible, which cannot reach the boiling point required by the equipment and cannot be attached to the surface of the substrate. It not only wastes time and manpower, but also causes the effective application of the evaporation material, the yield of the process becomes poor, and increases the cost burden of the raw material.  So choosing a good cauldron is a good place to start.  

Commonly used supports are: W, Mo, Ta, high temperature resistant metal oxides, ceramic or graphite crucible, but also pay attention to the possible reaction between the supporting material and the evaporation.  At present, organic matter and metal are the main materials for OLED evaporation. Crucible of the following three materials can be used.  

1). Tantalum (Ta)  

Tantalum is a hard blue-gray rare transition metal with high ducibility and chemical stability. It does not react with other compounds and has a melting point of 2996°C. It is mainly found in tantalite.  Tantalum is of moderate hardness and ductility and can be drawn into thin foil.  Its coefficient of thermal expansion is very small.  Tantalum has excellent chemical properties and is highly resistant to corrosion.  It does not react to hydrochloric acid, concentrated nitric acid and aqua regia, either under hot or cold conditions.  

Tantalum can be used to make vaporizers, as well as as electrodes for electron tubes, rectifiers, electrolytic capacitors, and as a minor component of alloys. It is also used in medicine to make thin sheets or wires to mend damaged tissues.  The corrosion resistance of tantalum mainly comes from the stable tantalum pentoxide (Ta2O5) protective film generated on its surface, which can resist oxidation well, so it can be used as the crucible material of vacuum evaporation parts.  Tantalum is used in many corrosion-resistant components.  

2). Special Graphite (Graphite)  

Graphite is a crystalline form of carbon.  Color is dark black, iron ink to dark gray.  Solution point 3652℃, boiling point 4827℃.  Soft, smooth surface, can be conductive, chemical inertness, reaction with acid, alkali, etc, have often been used to make into a crucible, as more kinds of less number of alloy melt is most suitable for use of crucible, since it has good high temperature performance and long service life, and be applied to large casting alloy smelting process and the fusion analysis of ore.  At the same time, graphite crucible has good liquid and gas permeability, high temperature resistance and dust suppression.

But because graphite crucible in use are basically in high temperature conditions, and there is a lot of air around, because it is easy to be oxidized.  And in smelting and casting, melting process, graphite crucible will be seriously corroded, reduce its service life.  In order to improve the service life of graphite crucible, silicon carbide and other materials can be added to the graphite crucible to enhance its corrosion resistance.  

3). Pyrolysis boron nitride (PBN)  

Pyrolytic boron nitride (PBN) is a hexagonal crystalline advanced ceramic material, ivory white, non-toxic, purity can reach 99.999%, acid and alkali resistance, oxidation resistance, good thermal conductivity, good surface density, high temperature resistance, no porosity, easy processing.  It is in high temperature, high vacuum conditions, the use of boron containing gas (BCl3 or B2H6) when the raw material, by chemical vapor deposition method, but because of B2H6 is highly toxic, BCl3 as the raw material.  The boron containing gas is pyrolyzed (1500~1800℃) and reacts with NH3 in a high-temperature reaction chamber to form boron nitride solid.  

The growth process of PBN material is similar to "falling snow", that is, the hexagonal BN snowflakes grown in the reaction continuously fall on the heated graphite matrix (core mold). With the extension of time, the accumulation layer thickens, that is, the shell of PBN is formed. When the PBN is removed, it is an independent and pure PBN component, and the PBN coating is left on it.  The same method can also be used to prepare PBN plates.

The high purity of PBN crucible is due to its preparation process without adding any sintering agent, so the use temperature under vacuum is up to 1800 degrees, and the use temperature under atmosphere protection is up to 2100 degrees (generally nitrogen or argon). Currently, it is most used in evaporation/molecular beam epitrization (MBE)/GaAs long crystal and other uses.  However, because of the slow deposition rate, the price of PBN crucible is quite expensive, and most of them are small size crucible.  

How to choose?  

It is to want to set out from reality of course, combine production craft requirement to undertake choice again.  For example, different evaporation materials can be matched with different crucible, because to heat up to the required boiling point of the material is not able to match each material crucible, so it has to be tailored according to the material characteristics, in order to achieve the purpose of full evaporation of the material.  In addition, crucible material density, thermal conductivity is also determined according to the actual production situation.  Therefore, the crucible for OLED evaporation generally needs customized production to achieve good results.