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.
Download the complete set of PCVD Diamond Growth application notes
Polysilicon Crystal Growth
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.
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
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MWx-AB - Aluminum Billets
MWx technology accurately measures high temperature billets where MW technology falls short. MWx pyrometer only works when looking at the side of the billet. No specific algorithm or offset is required – MWx-AB works across all alloys. Pyrometer output is set for 400-1100°F (200-600°C), but will only measure temperatures down to 575°F/300°C.