CVD and polysilicon crystal growing applications occur under highly controlled atmospheres. Measuring temperature inside a vacuum furnace can be particularly difficult as a direct contact measurement is not possible and non-contact temperature measurement has to deal with background reflections, plasma, energy, emissivity variation, and overall geometry issues. However, with the right wavelength selection, you can minimize errors from all of these interferences and produce a repeatable and accurate temperature measurement.
Diamond Deposition (Crystal Growth)
Diamonds can be artificially grown in a vacuum furnace under high-temperature and high-pressure conditions. These lab-grown diamonds use a diamond substrate (crystal) that is placed on top of a holder (typically molybdenum). A combination of methane (CH4) and hydrogen (H2) gas is added to the vacuum chamber and the gas is heated to a high temperature. Under these conditions ions from the gases dissociate and create a microwave plasma that surrounds the diamond in the target measurement area.
Temperature is a critical process control parameter during the CVD process and infrared pyrometers are often used to ensure quality and efficient process. The challenge for this application is that the plasma emits infrared energy at very specific wavelengths, depending on the chemical element creating the plasma. Therefore, it is critical to select a pyrometer with a wavelength that can view through the plasma so that you get a consistent temperature reading that is a measurement of the target diamond and not of the plasma.
The Williamson DW-24 wavelength set is the only dual-wavelength pyrometer that is capable of viewing through this plasma and providing a consistent temperature measurement reading during the Diamond CVD process.
Polysilicon Crystal Growth
Non-contact temperature measurement for silicon crystal growth and wafer processing can be challenging. There are a number of common application issues to deal with including:
Emissivity, Reflectivity, Transparency
- At temperatures below 600°C, the emissivity of silicon varies at wavelengths other than 0.9um
- The emissivity is less than 1.000 because silicon is both semi-transparent and also partially reflective
- Emissivity = 1.000 – (Transparency + Reflectiveness)
- Heating elements emit infrared energy and reflected heater energy is often the most significant source of pyrometer measurement error
- Plasmas emit infrared energy at specific wavelengths depending on the gas (argon, carbon, helium, hydrogen, nitrogen, oxygen) that is causing the plasma
However, with the right wavelength selection, these interferences can be minimized or eliminated.
A rigid light pipe is inserted into the chamber and through a hole in the wafer holder or heated chuck. The holder shields the pyrometer from viewing heater emissions.
SW Technology minimizes sensitivity to variation of emissivity and transparency at low temperatures.
The wafer must be opaque if quartz heaters are present above the wafer. For the annealing process, wafers are typically opaque.
CVD Crystal Growing
Silicon crystal growth is not an entirely clean process and often times the viewing windows on the vacuum chamber get dirty and introduce an interference. Dual-wavelength pyrometers eliminate this interference which would cause an errant reading for Two-Color or Single-Wavelength pyrometers. This is true for both the Siemens and the Czochralski (CZ) crystal growing techniques.
Consult With One of Williamson’s Temperature Experts
We would love to discuss your temperature measurement application with you.