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which are unsuitable for localized SSD characterization. Vickers indentations for Columns 1 – 5 and all 19 rows were made with HV0.5; the remaining Vickers indentations were prepared with HV0.6 given in Figure 1. The loading forces for HV0.5 and HV0.6 are 4.9 N and 5.9 N, respectively. The lateral spacing of the indentations is 1 mm to avoid cracks influencing each other. In order to achieve repeatable measurements and evaluations on the same position, numbers and micro holes are marked using an ultra-short pulsed( USP) laser( TruMicro 5050, Trumpf GmbH & Co. KG) on the sample surface.
2.2 SSD preparation by plasma jet etching
Plasma jet machining is a well-established technology for figuring and correcting various designs of optical elements mainly based on silicon containing substrates, such as silicon( Si), fused silica( SiO 2) or silicon carbide( SiC) [ 69 – 72 ]. The etching principle is based on chemical interactions between the reactive species generated in the atmospheric-pressure plasma jet discharge and the substrate material of optical glasses. Numerous research work published contain detailed information on PJE and related processes [ 69, 71, 73 – 75 ]. Hence, only some of the key facts of this technology are given here. The plasma jet source is based on a coaxial conductor system. The main process gases He, O 2 and CF 4 are supplied through the inner conductor whereas nitrogen( N 2) isfed peripherally to act as a shield gas. By applying microwave power( f = 2.482 GHz) provided by a microwave generator( PP3030, Trionplas Technologies GmbH) the gas mixture is ignited, by a high-frequency electric field at the nozzle. In thejet-likedischargetheCF 4 molecules undergo dissociation, excitation and ionization processes to form e. g. highly reactive fluorine species. Those reactive species interact with the sample surface causing the dry etching process. The plasma jet( PJ) source is mounted at a conventional CNC 5-axis motion system( PASO profispeed, PASO Präzisionsmaschinenbau GmbH). A variable material removal at different indentation positions was achieved using a wedgelike areal etch profile of 12.6 21.4 mm 2. The overlay of the generated indentation matrix and the wedge-like etch profile is depicted in Figure 1, with an increasing etch depth from row 1 to 19 as indicated by the purple color gradient.
This approach similar to taper polishing allows an efficient evaluation of depth-dependent surface morphology information compared to a layer-by-layer removal, though on the expense of statistical validation [ 1, 28, 36 ]. Wedge etching was consecutively performed eight times( denoted as PJE # 1 to # 8), while after each step the surface was characterized by means of topographic and tomographic measurement techniques. After eight PJE steps, a total depth of 110 lm was reached at the deepest point( see Fig. 1). The depth increments rise as the row number increases, so that only about 1 lm was etched in the finest( at row 1) per PJE step, while the largest steps( at row 19) were approx. 13.5 lm each.
2.3 Surface feature characterization
A digital microscope( VHX-7000, Keyence Deutschland GmbH) was used to examine the surface features at each
Vickers indentation. A customized ultra-high-resolution optical coherence tomography system( Thorlabs GmbH) was used to acquire tomographic 3D-scans of the surface and subsurface regions. Each Vickers indentation was measured three times – prior to PJE, and after PJE # 3 and PJE # 5, respectively. The OCT system operates at a central wavelength of 750 nm and provides an axial and lateral resolution of 1.2 lm in air. The interferometric signal is recorded with a spectrometer, processed with ThorImage Ò to gather 3D raw data and further evaluated using MA- TLAB Ò. The methods for evaluation and visualization of OCT results have been described in detail elsewhere [ 37 ]. In this work, a SSD area threshold of 0.1 % was applied to overcome the impact of a imaging artefact given by an outlier. This results in SSD depth values including 99.9 % of the pixels of the measured volume. The determination of material removal after PJE is based on areal form measurements using a non-tactile profilometer( CT 350S, Cyber Technologies GmbH). The indentations were analyzed after each PJE step with WLI( NPFlex, Bruker Corp.) to determine the evolution of indentation morphology as well as roughness parameters. Additional measurements using confocal microscope( lsurf explore, NanoFocus AG) were performed for columns 1, 4, 7 and 10( see Fig. 1) to overcome WLI measurement limitations due to high surface gradients and increased roughness observed for progressive etching depth. The field of view was 231 173 lm 2 for the WLI, and 320 320 lm 2 for the confocal microscope, respectively. These measurement areas ensured to capture only a single indentation. Using both WLI and confocal microscopy, the roughness parameter Sv( maximum pit height), and the void volume( VV) of the indentation were analyzed. The latter results from equation( 1), whereZ is the height value after removing the tilt of the measurement with respect to the surrounding area not influenced by the indentation.
VV ¼ X Zx ð; yÞxy: ð1Þ fZ0g
The matrix arrangement of the indentations and the unambiguous marking allowed to identify the individual measurement positions for all subsequent etching steps in a reproducible manner. The progression of both roughness parameters( Sv, VV) was analyzed as a depth-dependent SSD indicator. In addition, the evolution of the indentation morphology due to PJE was observed using an optical microscope( DSX 1000, Olympus Corp.) in dark field mode( fieldofviewof485 485 lm 2).
3 Results and discussion
3.1 Evolution of the Vickers indentation morphology: before and after PJE
First, the surface morphology of the Vickers indentations was analyzed using a digital microscope, as shown in Figure 2. The microscopic view reveals a similar morphology for all indentations and also exhibits some deviations due to the hard-brittle properties. Radial cracks and cone