Real-Time Process Monitoring by P-Polarized Reflectance Spectroscopy and Closed-Loop Control of Vapor Phase Epitaxy

摘要

Controlling and optimizing growth processes require improved methods of characterization and understanding of decomposition pathways and surface reaction kinetics. They also require the development of advanced nonlinear filtering and feedback control concepts. This contribution describes results on real-time optical monitoring of thin film growth processes by ppolarized reflectance (PR) utilizing a pulsed chemical beam epitaxy (PCBE) approach, where the growth surface is sequentially exposed to organometallic precursors. Under these conditions the surface reaction kinetics can be followed by analyzing a periodically (in composition and thickness) modulated surface reaction layer (SRL). This modulation can be captured in the PR-signals as a fine structure that is superimposed on the interference fringes produced by underlying growing film. The optical response is linked to the growth process via a reduced order surface kinetics (ROSK) model and integrated as a control signal in the implementation of filter and control algorithms for closed-loop controlled growth. The control concept has been applied for thickness and compositional graded multi-heterostructure Ga(x)In(1-x)P epilayers and validated by ex-situ post-growth analysis. This results in superior tracking of composition and thickness targets under closed loop controlled conditions when compared to films grown using pre-designed source injection profiles (open-loop conditions).