Performance Comparison of Crucibles for OLED Evaporation: Tantalum, Graphite and Boron nitride

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When it comes to display technology, in addition to the traditional LCD and LED liquid crystal display screen, OLED (organic light emitting diode) display technology has become a new mainstream.

OLED technology refers to the phenomenon of luminescence caused by carrier injection and recombination of organic semiconductor materials and light-emitting materials driven by electric field. As a solid-state self-illumination technology, OLED does not need to use the combination of LCD and LED backlight before, so it can make the TV thickness very thin, wider visual angle, lower power consumption, richer colors, and can be produced on different flexible substrate materials such as plastic, resin, etc., to achieve soft screen, so it is more and more popular by consumers.

In the manufacturing process of OLED, "evaporation" is both the essence and the difficulty. It means that in high vacuum, the material to be evaporated is heated by resistance or electron beam, and the OLED material is heated by crucible in the vacuum cavity to make the atoms evaporate and reach and attach to the surface of the substrate.

In OLED, in addition to luminous materials, even metal electrodes are steamed in this way, so the quality of the crucible directly affects the yield of the OLED process. If the unsuitable crucible is selected, the material in the crucible will not be evaporated completely, which cannot reach the boiling point required by the equipment, and cannot be attached to the surface of the substrate, resulting in the ineffective application of the evaporation material, the poor yield of the process and the burden of raw material cost.

Material of Crucibles For OLED Evaporation

The commonly used supports include W, Mo, Ta, high temperature resistant metal oxides, ceramic or graphite crucibles, etc., but it should also be noted that there may be reactions between the supporting materials and evaporation. At present, the OLED evaporation materials are mainly organic and metal, and the crucibles of the following three materials can be used.

Tantalum (Ta):

Tantalum is a kind of hard blue-gray rare transition metal. It has high ductility and high chemical stability. It does not react with other compounds and its melting point is as high as 2996 °C. it mainly exists in tantalite. Tantalum is of moderate hardness and malleable and can be drawn into thin wire foil. Its coefficient of thermal expansion is very small. Tantalum has excellent chemical properties and high corrosion resistance. It does not react to hydrochloric acid, concentrated nitric acid and "aqua regia" under both cold and hot conditions.

Tantalum can be used to make evaporating utensils, as well as electrodes, rectifiers and electrolytic capacitors of electronic tubes. it is often used as a secondary component of alloys and is medically used to make thin sheets or threads to repair damaged tissue. The corrosion resistance of tantalum mainly comes from the stable tantalum pentoxide (Ta2O5) protective film formed on its surface, which can resist oxidation very well, so it can be used as crucible material for vacuum evaporation parts. Tantalum is needed for many corrosion-resistant parts.

Graphite

Graphite is a kind of crystalline carbon. The colors are dark black, iron ink to dark gray. The melting point is 3652℃ and the boiling point is 4827℃. It is soft, smooth, conductive and inactive in chemical properties, and is not easy to react with acid and alkali, so it is often used to make crucibles, which is the most suitable crucible for melting of many kinds and small quantities of alloys. because of its good high temperature performance and long service life, it is used in large-scale alloy smelting casting process and ore melting analysis. At the same time, the graphite crucible has good anti-permeability of liquid and gas, can withstand high temperature and restrain the production of dust.

However, because the graphite crucible is basically under the condition of high temperature when it is used, and there is a lot of air around it, it is easy to be oxidized. Moreover, in the smelting, casting and melting process, the graphite crucible will be seriously corroded and its service life will be reduced. In order to improve the service life of the graphite crucible, silicon carbide and other materials can be added to the graphite crucible to enhance its corrosion resistance.

Pyrolytic Boron Nitride (PBN)

Pyrolytic boron nitride (PBN) is an advanced hexagonal ceramic material with ivory white, non-toxic and purity of 99.999%. Acid and alkali resistance, anti-oxidation, good thermal conductivity, good surface density, high temperature resistance, no pores, easy processing. It is made by chemical vapor deposition method using boron-containing gas (BCl3 or B2H6) as raw material under the condition of high temperature and high vacuum. However, BCl3 is mostly used as raw material because B2H6 is highly toxic. The boron-containing gas was pyrolyzed (1500-1800℃) and reacted 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 are constantly piled on the heated graphite matrix (mandrel). With the extension of time, the accumulation layer thickens, that is, the shell of PBN is formed, and the demoulding is an independent and pure PBN part, 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 because its preparation process does not need to add any sintering agent, so the service temperature is as high as 1800℃ in vacuum and up to 2100℃ in atmosphere protection (usually with nitrogen or argon). At present, it is most widely used in evaporation / molecular beam epitaxy (MBE) / GaAs crystal and other applications. However, due to the slow deposition rate, PBN crucibles are very expensive, mostly small size crucibles.

How to Choose?

Different evaporation materials can be matched with different crucibles, because to heat up to the boiling point required by the material, not all crucibles of each material can match. Therefore, it has to be tailor-made according to the characteristics of the material in order to achieve the purpose of full evaporation of the material.

In addition, the density and thermal conductivity of the crucible material are also determined according to the actual production conditions. Therefore, crucibles for OLED evaporation generally need to be customized in order to achieve good results.

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