High-pressure plasma etching up to 9 atm toward uniform processing inside narrow grooves of high-precision X-ray crystal optics

We developed a new etching technique using plasma generated at high pressure up to 9 atm. Operating at 9-atm pressure with a 30-μm-diameter wire electrode, we demonstrated the generation of well-ordered plasma at a narrow gap of ∼10 μm between the electrode and workpiece, and realized a high spatial resolution of <40 μm during processing. This technique should allow for the processing of high-precision X-ray crystal optical devices with compact and complex structures, such as a micro channel-cut crystal monochromator with an extremely narrow (sub-100 μm width) groove for realization of Fourier-transform-limited X-ray lasers with high intensity.


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ince the discovery of X-rays 1) with attractive characteristics, such as high transmissivities, atomic-scale wavelengths, and high sensitivities to inner-core electrons, it has been an indispensable tool for a broad range of applications from basic science to industry.][6][7][8] Recently, X-ray optics also played a key role in further innovation of X-ray sources.A specific example is the generation of intense and monochromatic X-ray free-electron laser (XFEL) [9][10][11][12][13] pulses through the so-called self-seeding principle [14][15][16][17][18] where an X-ray seed pulse with a narrow bandwidth is amplified through interaction with a low-emittance electron bunch in a long undulator.The key device in the self-seeding process is an X-ray monochromator, which extracts the narrowband X-ray seed from a broadband X-ray pulse emitted from the same electron bunch in an upstream undulator.The monochromator should meet the following requirements: collinear emission of a monochromatic X-ray pulse with the original broadband one and an ultrashort optical delay on the order of 100 fs for spatiotemporal overlap with the electron bunch, and efficient monochromatization without wavefront distortion for successful amplification of the seed.At Japan's XFEL facility, the SPring-8 Angstrom Compact free-electron LAser (SACLA), a unique monochromator called a micro channel-cut crystal monochromator (μCCM), was developed. 19) he μCCM is a monolithic silicon (Si) crystal block with a narrow groove of sub-mm width, which enabled the generation of intense XFEL pulses with a relative bandwidth of 0.06% at 9.8 keV by operating it in Si 111 reflection.16) Furthermore, a nearly strain-free Si(220) μCCM with ∼100 μm groove width was made available through a finishing method using plasma at atmospheric pressure (plasma chemical vaporization machining, PCVM [20][21][22][23][24][25][26][27] ) for removing the damaged layer on the inner-wall reflecting surfaces of the μCCM, and XFEL pulses with 0.007% bandwidth at 9 keV were successfully generated.26) Still, Fourier-limited XFEL generation has not been realized because it requires a narrower-band seed pulse created through a higher-order μCCM for which a narrower groove is necessary.26) For example, a groove width of 66 μm is needed for Si(400) μCCMs to achieve an optical delay of 200 fs at 10 keV.However, a strain-free μCCM with a sub-100 μm groove width was not realized because the available minimum groove width for the existing PCVM technique was limited to ∼100 μm.26) The limitation was attributed to the difficulty in generating stable plasma in the extremely narrow gap of ∼10 μm between an electrode and a workpiece, resulting in the distortion of surface roughness and flatness.One possible reason for the unstable plasma was the insufficient energy transfer from RF power supply into surrounding gas molecules because of the low number of molecules and less collisions with charged particles in the extremely narrow space.Plasma spread over a 100-200 μm region from the electrode was also a crucial issue for uniform processing on the narrow target area.
In this study, we propose a new PCVM technique that operates at high pressures beyond atmospheric pressure.The density of reaction gas molecules increases under highpressure conditions.In addition, the mean free path of charged particles and the diffusion of neutral radicals in the plasma are significantly limited, resulting in extreme localization of the plasma region as well as the processing area.Using an ultra-fine wire electrode with a 30-μm diameter and raising ambient pressure up to 9 atm, we realized wellordered plasma generation in an electrode-workpiece gap of ∼10 μm, and achieved extreme concentration of the plasma region that created a narrow removal footprint of 36 μm full-width at half maximum (FWHM).This new technique should enable damage-free processing of μCCMs with a groove width down to 50 μm.A brief introduction of the newly developed equipment for high-pressure PCVM and its processing characteristics are provided below.
Figure 1 shows a schematic illustration of the equipment used for the high-pressure PCVM.The processing chamber was designed to ensure no plastic deformation, even at high internal pressures of 1 MPa (∼10 atm).We used a commercially available wire made of SUS304 with a diameter of 30 μm as the electrode, which was fixed on a metal holder and kept under tension by a coil spring.The wire tilt was aligned along the work surface by positioning the screws on the insulator support.The electrode-workpiece gap was finely controlled using a vertical linear stage (Z-stage shown in Fig. 1) equipped on the wire holder through the insulator support.A horizontal linear stage (Y-stage) under the workpiece holder allowed the scanning of the plasma along the work surface.The entire wire and workpiece unit was placed on a base plate inside the chamber to maintain the relative position between the wire electrode and workpiece at variable internal pressures.The wire holder was connected to a 150 MHz RF power supply through an impedance matching circuit.The RF power supply was capable of pulse modulation, beneficial for avoiding wire breaks. 27)e investigated the processing characteristics of a highpressure PCVM with floating-zone (FZ) Si(100) wafers using the same material as the μCCM used for self-seeding at SACLA.The sample was cut into a 20 mm square.After evacuating the chamber to approximately 10 −1 Pa using a dry pump, the chamber was filled with a mixture of He and SF 6 with a volume ratio of He: SF 6 = 99.5:0.5 to the desired pressure.The pulse condition during processing was set to a frequency of 2 kHz and a duty ratio of 5%.After processing, the workpiece was removed by N 2 purging, and the surface profile was measured using a white light interferometric microscope (Zygo, NewView 7100).
First, the minimum RF powers required for maintaining plasma were examined for various internal pressures at an electrode-workpiece gap of 20 μm. Figure 2 shows the RF power (circle, left axis) and cross-sectional area of the static removal profiles at a dwell time of 10 s (square, right axis) as functions of the internal pressure.The RF power increased linearly with pressure because a higher electric field is required to provide charged particles with sufficient energy to ionize the surrounding molecules under high pressure with a short mean free path of the gas molecules.On the other hand, the cross-sectional area was almost independent of pressure, indicating that the amount of fluorine radicals generated in the plasma was constant under the condition of minimum RF power.Thus, we used the minimum RF power for subsequent experiments to compare the other processing characteristics.
Next, we investigated plasma uniformity and spatial resolution in processing.Figure 3 shows a conceptual illustration of PCVM processing with a wire electrode.Under conditions that allow well-ordered processing, the cross section of a static removal profile perpendicular to the wire appears Gaussian, as shown in Fig. 3(a).However, if the electrode-workpiece gap is too narrow, plasma is not generated between the lowest point of the wire and the workpiece, possibly because the number of collisions between the charged particles and gas molecules is insufficient.Instead, plasma is generated between the circumference of the wire and the workpiece and made inhomogeneous, forming a "W"-shaped removal profile as shown in Fig. 2(b).
For the quantitative evaluation of plasma uniformity, we measured the following parameters of the cross-sectional profiles: removal depth just below the wire (D B ), maximum removal depth (D M ), and their ratio which corresponds to the degree of plasma uniformity with a maximum of 1.0.The width of the profile at the removal depth of D 2 M / was defined as the processed width.Figure 4(a) shows the relationship between D R and the electrode-workpiece gap at pressures of 1 atm (circle), 3 atm (square) and 5 atm (triangle).A "threshold gap" was observed to achieve D R of 1.0 at each pressure, i.e., D R became less than 1.0 at gaps shorter than the threshold value.The threshold gap became smaller as the pressure increased, as expected: 65 μm, 25 μm, and 10 μm at 1 atm, 3 atm, and 5 atm, respectively.Note that D R was 1.0 for all the gaps we investigated (⩾10 μm) at pressures of 7 and 9 atm.
Figure 4(b) shows the dependence of the processed width on the internal pressure and typical cross-sectional profiles at 1 and 9 atm.Note that the gap and processing time were set to 20 μm and 10 s, respectively.At 1 atm, the processed width was 190 μm, i.e., the plasma had a lateral spread of ∼80 μm from the electrode of 30-μm diameter.In contrast, at 9 atm, the processed width was 36 μm with a plasma spread of only a few μm.The plasma spread tended to be inversely proportional to the pressure, explained by the distribution of the electric field intensity, which is inversely proportional to the distance from the electrode.The processed width does not asymptotically approach zero because the wire electrode has a diameter of 30 μm.
Finally, we performed surface processing by scanning the wire to evaluate the processing uniformity, because surface undulation on X-ray crystal optics disturbs the spatiotemporal properties of the reflected X-rays.We set a target of <100 nm peak-to-valley (PV) in an illumination area where the temporal elongation caused by the surface undulation is much less than 1 fs at 10 keV for Si 400 reflection.For comparison, we operated at 1 atm for conventional PCVM processing and 9 atm for the high-pressure PCVM proposed in this study.The gap was set to 20 μm.Besides, a wire scan that is too fast can make uneven profiles due to the acceleration and deceleration around the edge of the scan range, so the scan speed of the wire was set to a sufficiently slow value of 0.05 mm s −1 .The number of scans was adequately chosen to achieve a removal depth of 3 μm.Since the height of the groove of μCCMs currently employed at SACLA is 600 μm, the scan width was set to 400 μm except for both the top and bottom edges of 100 μm.The central 200 μm range was set to the illumination area, wider than typical beam size of 50-100 μm FWHM at SACLA.In addition, total processing time was 4 min.Note that there was no particular change in the appearance of the wire after the processing.
Figure 5 shows the cross sections of the removal profiles.The removal flatness at pressures of 1 and 9 atm was ∼250 nm PV and ∼40 nm PV in the central 200 μm range, The Japan Society of Applied Physics by IOP Publishing Ltd respectively.Notably, the lateral removal width was spread over ∼800 μm at 1 atm, wider than the groove depth assumed, indicating the removal flatness could be worse than this result.In contrast, the removal width at 9 atm was almost the same as the scanning width of 400 μm; thus, similar removal profiles are expected even on actual μCCMs.Furthermore, if we simply assume the removal flatness is proportional to the removal depth, it would be possible to increase the removal depth up to ∼8 μm, which is helpful for complete rejection of damaged layers on μCCMs that may be deeper than typical CCMs with mm-scale gaps.
In summary, we proposed the high-pressure PCVM technique with an ultra-fine wire electrode of 30-μm diameter for removing the damaged layer on the inner-wall surface of μCCMs with a channel width of <60 μm, which is used for self-seeding to generate high-intensity monochromatic XFEL.By increasing the pressure to a maximum of 9 atm, we stably generated plasma at an extremely narrow gap of ∼10 μm.In addition, the spread of the plasma region was suppressed at high pressures, with the spatial resolution in processing improved to almost the same as the electrode size.By applying this technique to actual μCCMs, the generation of highly monochromatic X-ray pulses as the seed while satisfying the constraint on the optical delay should be possible, providing an important step toward the realization of Fourier-limited XFELs.
In general, spatial resolutions in processing determine the upper limit of the spatial frequencies in figure errors, which can be corrected using numerically controlled PCVM techniques.The enhanced spatial resolution at high pressures demonstrated here could potentially enable more precise figure corrections in proven X-ray mirrors, 28) aspherical lenses, 29) drum-head X-ray beam splitters, 21,22) and quartz crystal substrates, 30) among others, as well as in highprecision X-ray crystal optical devices with more compact and complex structures.016001-4 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd

Fig. 1 .
Fig. 1.A schematic illustration of (a) the high-pressure PCVM equipment and (b) detailed view of electrode and workpiece part.016001-2

Fig. 2 . 3 ©
Fig. 2. Minimum RF peak power for maintaining plasma (red circles) and cross-sectional area of removal footprint (black squares) as the function of the internal pressure.016001-3

Fig. 3 .
Fig. 3.A conceptual illustration of plasma processing at (a) an adequate and (b) too narrow electrode-workpiece gap.

Fig. 4 .
Fig. 4. (a) Dependences of D R on gap at pressures of 1 atm (circle), 3 atm (square) and 5 atm (triangle).(b) Dependence of the processed width on pressure.Inset shows typical cross-sectional line profiles at 1 atm (red solid line) and 9 atm (black dotted line).

Fig. 5 .
Fig. 5.A cross-sectional line profile of the scanned area operated at 1 atm (solid red line) and 9 (dotted black line).