The Kyropoulos method (used to grow ultra-pure sapphire substrates for LEDs) involves melting a sapphire crystal in a heat-resistant crucible made from molybdenum or tungsten. A particularly critical stage during this process is when the sapphire crystal is extracted from the crucible. The easier the sapphire can be released from the crucible, the greater its yield and quality. If the surface of the crucible is too rough, the sapphire cannot be released or can only be partially released from the crucible’s wall, resulting in cracks and faults in the crystal. The crucible itself can also be damaged and have to be reworked or replaced earlier than planned.
To reduce this, Plansee has developed a pressed-sintered tungsten crucible with an ultra-smooth surface with surface roughness of less than 0.8 µm. The cycles run smoothly and the smooth surface is less susceptible to corrosion caused by the aggressive melted sapphire, extending surface life.
Plansee also sinters the crucibles at very high temperatures and a uniform temperature distribution, giving them a high material density of 93% and an homogeneous density distribution. Under high temperatures and rapid changes of temperature, the crucible can retain its shape.
To ensure that only flawless sapphire comes out of the crucible, Plansee has adopted an ultra-clean production process. Everything from the metal powder to the finished crucibles is manufactured in-house, so every step can be carefully monitored, resulting in crucible purity exceeding 99.97%.
