This paper presents a monolithic back-end-of-line integration method for phase change materials (PCMs) into foundry-manufactured silicon photonics, enabling the integration of novel materials without modifying existing photonic component libraries. The approach involves introducing a silicon nitride etch stop layer and low-loss oxide trench etching, allowing for the incorporation of functional materials without compromising device reliability. Two chalcogenide PCMs, Sb₂Se₃ and Ge₂Sb₂Se₄Te₁, were successfully integrated, offering compact phase and intensity tuning units with zero-static power consumption. The integration was demonstrated through the reduction of phase error in a push-pull Mach–Zehnder interferometer optical switch by 48% peak power consumption and the achievement of micro-ring filters with >5-bit wavelength selective intensity modulation and waveguide-based >7-bit intensity-modulation broadband attenuators. This foundry-compatible platform opens up possibilities for integrating other optoelectronic materials into future silicon photonic process design kits, advancing the field of heterogeneous silicon photonic integrated circuits.This paper presents a monolithic back-end-of-line integration method for phase change materials (PCMs) into foundry-manufactured silicon photonics, enabling the integration of novel materials without modifying existing photonic component libraries. The approach involves introducing a silicon nitride etch stop layer and low-loss oxide trench etching, allowing for the incorporation of functional materials without compromising device reliability. Two chalcogenide PCMs, Sb₂Se₃ and Ge₂Sb₂Se₄Te₁, were successfully integrated, offering compact phase and intensity tuning units with zero-static power consumption. The integration was demonstrated through the reduction of phase error in a push-pull Mach–Zehnder interferometer optical switch by 48% peak power consumption and the achievement of micro-ring filters with >5-bit wavelength selective intensity modulation and waveguide-based >7-bit intensity-modulation broadband attenuators. This foundry-compatible platform opens up possibilities for integrating other optoelectronic materials into future silicon photonic process design kits, advancing the field of heterogeneous silicon photonic integrated circuits.