Femtosecond Laser Machining of an X-ray Mask in a 500 Micron-Thick Tungsten Sheet

Abstract

Femtosecond laser material processing (FLMP) was used to make an X-ray mask in a 500 µm thick tungsten sheet without the use of any chemical etch methods. The laser produced an 800 nm wavelength at a 1 kHz repetition rate and a pulse width of 100 fs. The laser beam arrival at the tungsten sheet was synchronized to a computer numerically controlled (CNC) stage that allowed for motion in the XYZθ directions. The X-ray mask design was made using CAD/CAM software (Alphacam 2019 R1) and it consisted of linear, circular, and 45° angle features that covered an area of 10 mm × 10 mm. A total of 70 laser beam passes at a moderate laser energy of 605.94 J/cm² were used to make through-cut features into the tungsten sheet. The morphology of the top view (laser incident, LS) images showed cleaner and smoother cut edges relative to the bottom view (laser exit, LE) images. It was found that the size dimensions of the through-cut features on the LE surfaces were better aligned with the CAD dimensions than those of the LS surfaces. The focused laser beam produced inclined cut surfaces as the beam made the through cut from the LS to the LE of the tungsten sheet. The circular features at the LS surface deviated toward being oval-like on the LE surface, which could be compensated for in future CAD designs. The dependence of the CNC processing speed on the thickness of the etch depth was determined to have a third-order exponential decay relationship, thereby producing a theoretical model that will be useful for future investigators to predict the required experimental parameters needed to achieve a known etch depth in tungsten. This is the first study that has demonstrated the capability of using a femtosecond laser to machine through-cut an X-ray mask in a 500 µm thick tungsten sheet with no involvement of a wet etch or any other such supporting process.

Keywords: CAD/CAM; computer numerically controlled (CNC) motion; femtosecond laser material processing (FLMP); femtosecond laser pulse; laser ablation; tungsten X-ray mask; ultrafast laser processing; ultrafast lasers.

Figure 1

A simplified schematic showing the experimental setup used in the FLMP of a 0.5 mm thick tungsten sheet. The angle of incidence of the focused laser beam was perpendicular to the tungsten sheet surface.

Figure 2

A 2D top view of a CAD schematic representation of an X-ray mask to be machined in a 0.5 mm thick tungsten substrate. Dimensions are in mm units. The width of the channels, called channel throat, was 0.13 mm, while the distance between the two channel features, called pore body, was separated at lengths ranging from 0.17–0.38 mm. The diameter of all of the circular features was 0.25 mm. Notations such as (L1) and (L2) simply denote the dimensions where there were comparison data on the CAD, laser incident surface side (LS), and laser exit (LE). See a further discussion in Table 1.

Figure 3

(A–D) The top view images (laser incident (LS) surface). (E–H) The bottom view images (laser exit (LE) surface) of the FLMP X-ray mask. A and E are the optical camera images that show the total area view, while the rest of the images (B–D,F–H) were taken with an SEM that showed a smaller area view. Images (A–D) are representative of top view and (E–H) are representative of bottom view images. The arrows are pointing to morphological regions of interest, including circles, ovals, saw-like shapes, and machined debris. The trapezoid geometry in (D) illustrates the cross-section of the channel, where the long and short base lines represent the LS and LE length, respectively. The scale bar on (D) is for all images except (A,E).

Figure 4

SEM images taken at 90° to the cut surface of the (A) as-purchased tungsten sheet in comparison to that of the (B) FLMP cut surface. The LS surface and LE surface arrows point to the laser incident and laser exit regions across the 0.5 mm thick tungsten sheet. The laser cut direction is from the LS to the LE surfaces, as indicated by the cut direction arrow. The LS region had a smooth cut edge relative to the LE region (dashed rectangle).

Figure 5

(A) A line profile scan across multiple FLMP-etched depths into tungsten sheet at a constant fluence of 353.47 J/cm² and at different CNC processing speeds. A pitch of 5 µm was used, and the different speeds in mm/s are shown below each depth. (B) An exponential decay fitting based on measured experimental data points (black squares) as a function of the CNC processing speed. The theoretical fitting results are given on the graph.