For the first time, a spectroscopic ellipsometer (SE) has been integrated as an in situ layer thickness sensor into a vertical hatch furnace for industrial LPCVD layer deposition. A SOPRA MOSS-OMA SE was selected because of its high accuracy and versatility. In the vertical furnace, the SE can be used for in situ sensing of the layer growth as well as for post-process control of the batch. The adaptation of the SE to the furnace was performed with only minor modifications to the furnace geometry. The light beam of the SE is guided through the base plate into the furnace tube and directed onto the wafer by quartz glass prisms operating in total internal reflection mode. This arrangement introduces a well-defined additional phase shift in the polarization state of the light, which can be calculated and subtracted from the measured phase shift. The system was used in the first step to determine the optical reference data for crystalline silicon, silicon oxide and silicon nitride as a function of temperature. These data were implemented in the refractive index library of the in situ SE for endpoint monitoring and control of layer composition in the second step. The arrangement of the in situ SE also enables post-process measurements on selected wafers of the hatch during the unloading sequence. In situ as well as post-process data were used by the furnace for immediate and automated correction of parameter settings. Rapid process optimization and real-time control of integrated multilayer processing have been demonstrated to he the major benefits of the novel real-time SE technique.