MOCVD - LED Chips
GaN based LEDs are nowadays widely used in both indoor and street lighting, TFT LCD display backlighting, and automobile applications.
The LED chips are typically grown on sapphire or SiC substrates. One of the most important process parameters in the growth of the chips is probably temperature.
Indium incorporation is highly process-temperature-dependent (with III/V ratio being the other important factor). It is therefore very desirable to control the substrate temperature during Multiple Quantum Well (MQW) growth. The traditional solution for temperature instrumentation on the process side (wafer side) was Emissivity Corrected Pyrometer (ECP). However, with the typical wafers used in this application, the ECP measures the susceptor temperature instead of the substrate temperature. That works quite well if the front side of the substrate is polished and the substrate remains flat during process.
During real processes, the wafer can lift slightly on one side in some reactors and can bow in most reactors. That creates temperature non-uniformity on the substrate. Unfortunately, the non-uniformity cannot be detected by ECP in case of IR-transparent substrates. The measurement from an ECP therefore could mislead the MOCVD reactor user in true substrate temperature.
In recent years, Patterned Sapphire Substrate (PSS) has been widely adopted in production. With the patterned surface, an ECP loses its accuracy in reflectance measurement and, in turn, loses the accuracy in temperature measure even on the susceptor.
By taking advantage of the opaqueness of the GaN epi layer, a pyrometer working near or below 400 nm would be able to measure the substrate temperature accurately. The UV pyrometer concept was originally proposed by Sandia National Lab and the commercialization only became very successful in recent years.
LumaSense offers a proven UV pyrometer that is also affordable. The data from our UV 400 shows a very linear temperature – PL relationship, which is not available with ECP. The UV 400 is the solution that enables yield enhancement and quality control in the very competitive LED chip market. LumaSense can help customers with different levels of implementation needs.
True wafer temperature measurement instead of susceptor temperature measurement
Accurate temperature measurement on PSS
Linear growth temperature – wavelength relationship
Enables yield improvement
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