Development of a model for ultra‐precise surface machining of N‐BK7® using microwave‐driven reactive plasma jet machining

Abstract

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.

A novel Deal–Grove‐inspired model for fluorine‐based plasma jet etching of borosilicate crown optical glass

“…In other words, the surface temperature on a position x within the etched line is assumed to be constant. In the real experiment, the spatial temperature exhibits, however, a lateral distribution in the x, y plane that resembles a comet‐like footprint, as shown, for example, in the study reported by Kazemi et al [ 24 ] Thus, each position x experiences an increase of temperature up to the value taken in the model, and subsequently a decrease, when the plasma jet passes a certain position y . As the model disregards this temperature variation, the computed depth is larger than that observed in the experiment.…”

“…The plasma jet as a source of reactive species imposes a characteristic spatial distribution of fluorine flux on the surface, depending on the plasma parameters. Furthermore, according to our former investigation, [ 23,24 ] the heat flux of the plasma leads to the development of a spatio‐temporal surface temperature distribution. Both effects lead finally to the characteristic bell‐shaped profiles of the residual layer thickness or etching depth, which can be approximated by a Gaussian function as follows: …”

“…Hence, the application of fluorine-based reactive PJM to surface machining of N-BK7 is challenging and requires a comprehensive model to determine the time-varying nonlinearity of the material removal rate. [24] For this purpose, first, the exact interactions between plasmagenerated active particles (i.e., fluorine) and the N-BK7 surface atoms should be clarified. The principle of PJM of N-BK7 by fluorine-based plasma jet is shown in Figure 2.…”

Advances in precision freeform manufacturing by plasma jet machining -INVITED

“…The reason for this is a structural change of the residual layer from a densely closed layer occurring at room temperature to a porous layer found at elevated temperature. Thus, the fluorine atoms can penetrate the layer reaching the N-BK7 interface to proceed etching [2,3]. Although the absolute values of etching rate gradually decrease with increasing layer thickness, such a homogeneous behaviour is more suitable to be taken into account in a dwell-time based machining algorithm.…”

Laser-induced reactive microplasma for etching of fused silica

“…It was shown that the excitation caused by APPJ can be induced by RF or microwave power. The interaction of APPJ with substrates have been experimentally [7][8][9] and theoretically [10] investigated. It has been demonstrated that APPJ processing can be used to etch a variety of different materials such as Si [11], technical glass [12], and SiC [13].…”