An apparatus and corresponding method are disclosed that uses one or more optical fibers in a susceptor that monitor radiation emitted by the backside of the susceptor. The optical fibers are filtered and converted into an electrical signal. A control system is used to maintain a constant wafer temperature by keeping the electrical signal constant during the deposition cycle. This overcomes problems caused by varying wafer temperature during non-selective epitaxial and poly-silicon growth on patterned wafers at low temperatures and reduced pressure.