Comment by Panoramix

It's about the thermal conductivity.

Helium has 150mW/mK vs Argon ~18mW/mK so you can't replace it.

The only alternatives to Helium are Neon, which is 3x worse and much more expensive, and hydrogen. However, hydrogen is flammable so it's a very bad idea to use it in a fab which has extremely poisonous gases and needs a cleanroom environment. A fire would ruin your whole factory and kill your engineers.