In this paper, extensive studies are conducted as key to overcoming several challenging limitations in applying fluorine‐based reactive plasma jet machining (PJM) to surface machining of N‐BK7®, particularly regarding the manufacture of freeform optical elements. The chemical composition and lateral distributions of the residual layer are evaluated by X‐ray photoelectron spectroscopy and scanning electron microscopy/energy‐dispersive X‐ray spectroscopy analysis aiming at clarifying the exact chemical kinetics between plasma generated active particles and the N‐BK7 surface atoms. Subsequently, a model is developed by performing static etchings to consider the time‐varying nonlinearity of the material removal rate and estimate the local etching rate function. Finally, the derived model is extended into the dynamic machining process, and the outcomes are compared with the experimental results.
In this study, a fluorine‐based reactive plasma jet is investigated as a promising tool for ultraprecise surface machining of N‐BK7®. Plasma‐generated particles react with an N‐BK7 surface to create volatile and nonvolatile compounds. The desorption of volatile compounds results in an etched surface, whereas nonvolatile compounds form a residual layer in the etched area, causing unpredictable effects on the etching rate. Surface temperature treatment is proposed to improve the machining procedure with respect to deterministic material removal, leading to predictable results. It is shown that, at an elevated surface temperature, the residual layer properties are modified in favor of improved etching performance. The etching behavior of N‐BK7 is compared with fused silica to verify the optimality of the obtained results.
The Deal–Grove model is a state‐of‐the‐art approach proposed for describing the thermal oxidation of silicon and the oxide thickness over time. In this study, the Deal–Grove concept provided the inspiration for a mathematical model for simulating plasma jet‐based dry etching process of borosilicate crown glass (N‐BK7®). The whole process is contained in two so‐called Deal–Grove parameters, which are extracted from experimental data including local etching depth and surface temperature distribution. The proposed model is extended for the evolution of dynamic etch profiles, and the obtained results are validated experimentally. By establishing such a model, it is possible to predict the effect of the residual layer and surface temperature on the evolution of local etching depths over dwell time.
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