Femtosecond-laser sharp shaping of millimeter-scale geometries with vertical sidewalls

As femtosecond (fs) laser machining advances from micro/nanoscale to macroscale, approaches capable of machining macroscale geometries that sustain micro/nanoscale precisions are in great demand. In this research, an fs laser sharp shaping approach was developed to address two key challenges in macroscale machining (i.e. defects on edges and tapered sidewalls). The evolution of edge sharpness (edge transition width) and sidewall tapers were systematically investigated through which the dilemma of simultaneously achieving sharp edges and vertical sidewalls were addressed. Through decreasing the angle of incidence (AOI) from 0° to −5°, the edge transition width could be reduced to below 10 µm but at the cost of increased sidewall tapers. Furthermore, by analyzing lateral and vertical ablation behaviors, a parameter-compensation strategy was developed by gradually decreasing the scanning diameters along depth and using optimal laser powers to produce non-tapered sidewalls. The fs laser ablation behaviors were precisely controlled and coordinated to optimize the parameter compensations in general manufacturing applications. The AOI control together with the parameter compensation provides a versatile solution to simultaneously achieve vertical sidewalls as well as sharp edges of entrances and exits for geometries of different shapes and dimensions. Both mm-scale diameters and depths were realized with dimensional precisions below 10 µm and surface roughness below 1 µm. This research establishes a novel strategy to finely control the fs laser machining process, enabling the fs laser applications in macroscale machining with micro/nanoscale precisions. • A fs laser sharp shaping approach is presented for achieving micro/nanoscale precisions in macroscale machining. • Sharp edges and vertical sidewalls are simultaneously achieved by AOI control and parameter compensations. • Through adjusting AOI, the edge transition width is reduced to below 10 µm but at a cost of increased sidewall tapers. • Through parameter compensations, non-tapered sidewalls are produced for various macroscale geometries with <10 µm precisions and <1 µm surface roughness. • Sidewall tapers are tuned from positive to negative by controlling fs laser ablation via parameter compensation.


Introduction
Higher precisions are a major goal being continuously pursued in most modern manufacturing techniques. Nowadays, micro/nanoscale precisions have been achieved through different methods but primarily for microscale geometries [1][2][3]. Machining macroscale geometries above millimeters (mm) while keeping precisions at micro/nanoscales is still a remaining challenge.
Femtosecond (fs) laser machining has attracted increasing interest in various fields due to its unique advantages of being noncontact, material-independent, flexible, and having low thermal effects. As fs lasers of high pulse energies and repetition rates are emerging, their machining capabilities are being continuously pushed to new boundaries including smaller features, larger scales, higher resolutions, and higher efficiencies [4][5][6][7]. Significant advances have been realized in machining extremely small geometries (e.g. below 50 nm [8,9]). Nevertheless, achieving micro/nanoscale precisions in both shapes and dimensions with fs lasers when machining macroscale geometries is more challenging in critical industrial fields, such as the manufacturing of injection nozzles in engines [10,11] and cooling holes on turbine blades [12,13].
Over the past decade, the ability to flexibly control the scanning of laser beam has significantly improved, in particular, due to the development of five-axis (5-axis) scanners ( figure 1(b)), permitting tuning of sidewall tapers from positive (i.e. entrances > exits) to negative (i.e. entrances < exits) [10,23,24]. Our preliminary experiments showed that sidewalls with near-zero tapers can be produced even at an AOI of 0 • when proper fs laser conditions were used through a 5-axis scanner. However, poor-quality edges with rough chamfers at the entrances are yet to be addressed (figures 1(e), (h) and (k)), particularly when machining deep holes (⩾1 mm) where the movement of fs laser focal spots along the depth direction (i.e. the Z-direction) is inevitable. Strategies to simultaneously realize both vertical sidewalls and sharp edges are highly demanded for achieving micro/nanoscale precisions in macroscale machining.
In this research, we developed an fs laser sharp shaping approach (figure 1(c)) to machining macroscale holes of various geometries using a 5-axis scanner. The underlying mechanisms for the formation of poor-quality edges and sidewall tapers were revealed. For the first time, the fs laser ablation behaviors in actual manufacturing were flexibly controlled, through which a parameter-compensation strategy was developed to improve the sidewall and edge qualities in fs laser machining. Through tuning the AOI and implementing the parameter compensation (by gradually decreasing the scanning diameters and using optimal laser powers), we simultaneously achieved vertical sidewalls as well as sharp hole entrances and exits (figures 1(f), (i) and (l)). Apart from round holes, geometries of different shapes with both mmscale lateral dimensions and depths were also achieved, with dimensional precisions below 10 µm and surface roughness below 1 µm. In addition to AOI which is most commonly used to tune tapers, this research indicates that controlling the fs laser ablation behaviors via parameter compensation can also realize tuning of sidewall tapers from positive to negative. This strategy demonstrates the capability of fs lasers in macroscale machining in addition to its prominent advantages in micro/nanoscale manufacturing.

Experiments
The fs laser machining was conducted using an Amplitude Tangor laser, which operates at a central wavelength of 1030 nm and a pulse duration of ∼ 408 fs. As a widely used engineering material, SAE 304 stainless steel was selected as a typical example in this research. The specimens were square plates measuring 1 × 1 in 2 with a thickness of 0.91 mm. The linearly polarized laser beam firstly passed through a beam conditioning unit (SCANLAB GmbH) and was transformed to circular polarization. Subsequently, the circularly polarized laser beam was scanned and focused to a spot of ∼20 µm in diameter via a precSYS 5-axis micromachining system (SCANLAB GmbH), which can modify the AOI in a range of −7.5 • to +7.5 • . During the laser machining, compressed air was continuously fed through a nozzle attached to the precSYS 5-axis system as an assisting gas to eject residues from laser ablation.
We developed a two-step approach to machining geometries with macroscale dimensions (e.g. holes with both a 1 mm diameter and a 1 mm depth) with clean edges, as shown in figure 2. In Step 1, the fs laser was rotated along spiral circles with a speed of 50 Hz (rounds per second) in the X-Y plane. At each depth position, the entire-hole-area was repeatedly filled with spiral circles for 25 times (Process 1). Then, the fs laser focal spot was forwarded 100 µm along the Z-direction to the next depth position (Process 2). After reaching the targeted depth, the laser focal spot was moved back to the entrance to conduct another machining cycle (i.e. Processes 1 and 2). In total, four machining cycles were conducted in Step 1 and, thus, the entire-hole-area was filled with spiral circles for 100 times (25 × 4) at each depth position. Herein, the target depth was set to be ∼400-500 µm in Step 1, which was sufficient for machining through 1 mm-deep holes. In Step 2, helical scanning was performed continuously throughout the entire sidewalls (with a revolving distance of 1 µm and a rotating speed of 200 Hz) and repeated for 250 times.
Step 1 was designed to achieve complete material removal to facilitate Step 2.
Step 2 was performed as a high-speed refining process to realize smoother sidewalls and sharper, cleaner edges with fewer depositions and debris.
In Step 2, the helical scanning diameters were set to be 20 µm smaller than the designed diameters. The scanning diameters in Step 1 were either consistent at different depth positions or gradually reduced from the entrances to deeper positions. Regardless of the specific diameter values, the maximum spiral circles in Step 1 were set to be 45 µm smaller than the scanning diameters in Step 2 at the same depth position throughout the entire depth. The preliminary experiments identified that a pulse repetition rate of 110 kHz and a laser power below 9.8 W can render the machining of high-quality holes of 1 mm diameter and 1 mm depth. Therefore, the pulse repetition rate was kept constant at 110 kHz, while the laser average power was adjusted in a range of 2.5-9.8 W to achieve higher machining precisions. The SEM images were collected by an FEI Helios NanoLab 660 Dual Beam scanning electron microscope. The dimensions of the geometries machined at both their entrances and exits were measured from their SEM images using the ImageJ software. The three-dimensional (3D) images of the machined geometries were collected using a Keyence VK-X200K laser scanning microscope. Schematics of the two-step machining approach: (a) step 1, the complete material removing process via laser rotating along spiral circles; (b) step 2, the high-speed refining process via continuous helical scanning.

Mechanism underlying the formation of poor-quality edges
The mechanism underlying the formation of poor-quality edges was first investigated. For holes shallower than the laser focal depth, the entire hole can be machined without forwarding the laser focal spot along the Z-direction, leaving the machined hole free from the influence of laser defocusing. However, machining deeper holes beyond the machining capability of the fs laser within a single focal plane requires the Z-direction movement of the laser focal spot, resulting in the re-machining of the entrances under defocusing conditions and consequently the poor-quality edges.
To verify this, we gradually forwarded the fs laser focal spot from the top surfaces of the samples (Z = 0 µm) to a depth position of 400 µm with the AOI fixed at 0 • . As presented in figures 3(a1)-(a3), the entrances were initially sharp when laser machining was conducted at the surfaces. As the laser focal spot moved deeper, rough edges appeared at the entrances with increased chamfers, as shown in figures 3(b1)-(c3), indicating the occurrence of the laser machining at defocusing conditions. More detailed results for Z = 0, 100, 200, and 400 µm are shown in figure S1 (available online at stacks.iop.org/IJEM/3/045001/mmedia).
Since beam divergence is the main consequence of laser defocusing, the poor-quality edges are expected to be reduced after correcting beam divergence. As a comparison, the maximum AOI provided by the 5-axis micromachining system (i.e. AOI = −7.5 • ) was used when the laser focal spot was forwarded to a depth of 400 µm. As shown in figures 3(d1)-(d3), clean and sharp entrances are achieved without noticeable chamfers, confirming the dominant role the beam divergence plays in causing the defects on edges in macroscale machining.
To examine the decrease in defects on edges with AOI, round holes of the same diameter were machined using the same laser parameters and forwarding depth (Z = 400 µm) but with AOIs being varied by a step of 1 • . The low-magnification images in figure 4 indicate that an obvious ring-like feature appears on the entrances at an AOI of −1 • , decreases at an AOI of −3 • , and becomes unnoticeable at an AOI of −5 • .
The zoomed-in images in figure 4 clearly show that the edges become sharper and smoother as larger AOIs are used. More detailed results for AOI = −1 • , −2 • , −3 • , −4 • , and −5 • can be found in figure S2.

Influence of AOI on both edge quality and sidewall taper
To quantify the influence of the AOI, the entrance and exit diameters of the holes machined at different AOIs were measured at various orientations and averaged, as shown in figure 5(a). The edge transition widths (edge widths for short, i.e. the differences between the outer and inner radii of the entrances) and the sidewall tapers were analyzed, as plotted in figure 5(b). The edge width was used in this research to evaluate the defects on the edges of entrances. With larger AOIs, the outer diameters of the hole entrances obviously decreased while the inner diameters of entrances showed only slight changes (figure 5(a)). Consequently, the edge widths evidently decreased from above 50 µm at an AOI of 0 • to below 10 µm at an AOI larger than −5 • (figure 5(b)). Meanwhile, the exit diameters obviously increased (figure 5(a)), resulting in larger sidewall tapers (figure 5(b)).
The cross-sectional schematics in figure 5(a) illustrate the evolution of the hole shapes as larger AOIs are used. Vertical sidewalls (i.e. the inner diameters of the entrances almost equal the exit diameters) can be achieved at small AOIs (e.g. AOI = 0 • ). However, obvious edges were produced at the entrances. Controlling the AOI can reduce edges but results in increased sidewall tapers. It is difficult to simultaneously control the outer and inner entrance diameters and the exit diameters to the same value within an error range of 10 µm. Therefore, producing vertical sidewalls with sharp edges cannot be realized by simply controlling the AOI.
To solve the dilemma of difficulties in achieving sharp edges and vertical sidewalls simultaneously, the fs laser ablation behaviors were further analyzed. As illustrated in figure 6(a), the overall ablation can be considered to be composed of the vertical and lateral ablation (i.e. along the depth and radial directions for round holes) at oblique incidences. The vertical ablation is for machining deeper holes while the lateral ablation is for increasing hole diameters. When a constant scanning diameter is used at oblique incidences, the laser ablation occurs vertically and laterally at the same time, causing cone-shaped holes with negative sidewall tapers. Although the vertical ablation is always the major part of the overall ablation at small AOIs, the lateral ablation increases at larger AOIs, resulting in larger sidewall tapers. Since the AOI is necessary to achieve sharp edges, preventing lateral ablation at oblique incidences is the key to vertical sidewalls. Accordingly, two specific parameters were carefully adjusted to compensate for lateral ablation (i.e. the scanning diameter and the laser power), with a parameter-compensation strategy developed for sharp shaping geometries with vertical sidewalls. The edge widths are nearly identical at AOIs of −5 • , −6 • , and −7 • but the sidewall tapers increase continuously ( figure 5(b)). Therefore, an AOI of −5 • was selected as the optimal value in this research for suppressing defects on edges, and it is used in the following discussions. According to the manual of the precSYS system, the typical divergence angle of focused beam is ∼0.08 rad (i.e. ∼4.6 • ), which is close to the optimal AOI value extracted from the experiments above. It indicates that the defects on the edges of entrances were largely determined by the beam divergence and also influenced by actual manufacturing processes (e.g. shapes and dimensions of geometries, machining approaches, laser parameters, etc).

Parameter-compensation strategy
Gradually decreasing the scanning diameters along the depth (i.e. the laser scanning routes are programmed as inverted cones, as shown in figure 6(b)), is defined as the scanning-diameter compensation (diameter compensation) in this research. The difference between the maximum and minimum programmed scanning diameters is the amount of diameter compensation. In addition, using the optimal laser power is defined as the laser-power compensation (power compensation), as shown in figure 6(c). Both scanning-diameter and laser-power compensations were applied to reduce lateral ablation and ensure that the majority of laser ablation occurs vertically. The scanning-diameter compensation reduces lateral ablation through conducing off-focus ablation at the sidewalls, while the laser-power compensation is based on the dependence of ablation on laser fluences.
As shown in figure 6(d), the scanning-diameter compensation effectively reduces the exit diameters, making them closer to the entrance diameters. However, the effect of scanningdiameter compensation weakens when using larger diameter compensations. When a 100 µm scanning-diameter compensation was used, the difference between the exit and entrance diameters narrowed more than 20 µm. However, when the scanning-diameter compensation was further increased from 100 to 400 µm, the difference between the exit and entrance diameters narrowed less than 20 µm. The limitation of the scanning-diameter compensation can be more clearly seen from the evolution of the sidewall tapers shown in figure 6(e). The sidewall taper decreased from ∼3.6 • without diameter compensation to ∼2.7 • with a 100 µm diameter compensation but failed to further decrease to below 2.0 • even with a 400 µm diameter compensation. It is suggested that the effect of off-focus ablation in reducing lateral ablation is limited, and, hence, scanning-diameter compensation cannot completely eliminate sidewall tapers.  Considering that the laser power has more influence on fs laser ablation, laser-power compensation using optimal laser powers was further implemented. So far, a laser power of 9.8 W was used. When lower laser powers of 7.1 and 5.6 W were used, the exit diameter decreased almost linearly and eventually approached the entrance diameters within an error of 10 µm with a fixed scanning-diameter compensation of 400 µm (figure 6(d)). Accordingly, the sidewall tapers were reduced from 2.3 • to 1.3 • and then 0.3 • at laser powers of 9.8, 7.1, and 5.6 W, respectively (figure 6(e)). The success in achieving nearly non-tapered sidewalls through parameter compensation proves the validity of the analysis on the fs laser lateral and vertical ablation behaviors described above.
In all scanning-diameter and laser-power compensation tests, the outer and inner diameters of the entrances remained at steady levels with a constantly small difference between them (figure 6(d)). As a result, the edge widths were kept below 10 µm independent of the applied parameter compensations (figure 6(e)). Therefore, the AOI control combined with the parameter compensation provides a versatile solution to simultaneously control the edge and sidewall qualities, with sharp edges and vertical sidewalls achieved simultaneously. Here, a 400 µm diameter compensation and a 5.6 W laser power were applied to machine holes with 1 mm diameters and 1 mm depths. The parameter-compensation strategy can be flexibly adjusted for machining macroscale holes of other diameters and depths.

Flexibility of the parameter-compensation strategy
More details on the influence of laser-power compensation on dimensional precisions are presented in figure 7. Laser powers of 9.8, 8.2, 7.1, 5.6, 4.7, 3.4, and 2.5 W (at a repetition rate of 110 kHz) were separately used to machine the holes of 1 mm diameter and 1 mm depth, with a fixed scanning-diameter compensation of 400 µm. As the laser power decreased from 9.8 to 2.5 W, the exit diameters decreased obviously and monotonically, while the outer and inner diameters of the entrances showed no apparent changes. As a result, the exit diameters changed from above to below the entrance diameters as the laser power decreased from higher to lower than 5.6 W, and the profiles of the holes changed from cones to inverted cones. The laser power of 5.6 W produced the least-tapered sidewall for machining 1 mm diameter and 1 mm deep holes at an AOI of −5 • .
The change of laser power shows little influence on the entrance diameters. At the entrance, the effect of the overall ablation being divided to vertical and lateral portions is insignificant. Therefore, the entrance diameters are mainly determined by the programmed laser scanning routes and slightly influenced by the laser power. The edge widths also stay at a steady level once the AOI is fixed. As the laser machining goes deeper, the effect of lateral ablation accumulates, resulting in negatively tapered sidewalls. When lower laser powers are used as parameter compensations, the lateral ablation decreases more than the vertical ablation since the former is only a small portion of the overall laser ablation (figure 6(c)). Accordingly, the proportion of lateral versus vertical ablation is reduced, producing vertical sidewalls and even invertedcone holes.
A logarithmic dependence of fs laser ablation on laser fluences (determined by laser power in this research) has been widely identified [25][26][27][28]. A logarithmic fitting was also made between the exit diameter and the laser power, which matches well with the experimental data. The difference between the inner diameter of the entrance and the exit diameter was further logarithmically correlated with the laser power. Both logarithmic relationships confirm the dominating role that ablation behaviors play in the parametercompensation strategy presented above. The logarithmic dependence of the exit diameters on laser power also leads to the superior effectiveness of laser-power compensation over scanning-diameter compensation in controlling sidewall tapers.
Further, holes with diameters of 1050 and 950 µm (i.e. 50 µm larger and smaller than those holes above) were machined to show the stability of the parameter-compensation strategy. As highlighted in the green circles in figure 7, the outer and inner diameters of the entrance and exit diameters increase or decrease synchronously by ∼50 µm and remain almost equal to each other. It indicates that the parameter compensation can precisely control the hole diameters and be generally used to machine macroscale holes of various sizes. Past investigations on fs laser ablation behaviors were primarily conducted through measuring basic features (e.g. craters and lines) ablated on solid surfaces under various laser conditions for ablation-depth and ablation-threshold analysis [27,28]. This study succeeded in controlling the fs laser ablation behaviors within actual manufacturing-in particular, the sidewall quality control-and explores the flexibility in controlling both the fs laser energy input and laser-matter interactions to realize high-precision machining.

Examination of surface quality
The surface finish inside the round holes after completing both Steps 1 and 2 was also examined through SEM at different depth positions. As shown in figure 8(b), the top part of the sidewall is clean and smooth except for the remaining roughness at the entrance within a depth of 50 µm and a lateral width of 10 µm. The top part of the sidewall is covered by discontinuous submicron ripples together with some submicron particles (figure 8(e)), which are both typical structures induced by fs laser processing of metal surfaces [29,30]. The middle part of the sidewall is more uniform (figure 8(c)), mainly covered by submicron particulates (figure 8(f)). Since the laser focal spot was only forwarded to a depth of 500 µm in Step 1 to save the entire machining time, the bottom part of the sidewall was not uniformly ablated by the fs laser focal spot, resulting in light tracks of fs laser propagation along the Z-direction ( figure 8(d)). In spite of this, figure 8(g) indicates that the tracks are also covered by the fs laser induced submicron features, showing no microscale surface roughness.
The formation of nanoscale features, particularly ripples, on sidewalls of fs laser machined holes and cavities has been previously reported [10,11,31,32]. The orientation of ripples is mainly determined by the polarization conditions of fs lasers and also affected by other machining conditions. In this research, it is believed that the laser rotating along spiral circles in Step 1 primarily accounts for the formation of ripples, while the continuous and high-speed refining in Step 2 accounts for both the formation of submicron particulates and the discontinuity of the ripples. Since the laser focal spot was only forwarded to a depth of 500 µm in Step 1, ripples were more obvious on the top part of the sidewall and remained there after Step 2. From top to bottom of the sidewall, the influence of Step 2 becomes more obvious, producing more prominent particulate features. Regardless of the surface features, the quality examinations verify that the parameter-compensation strategy using the 5-axis scanner can machine mm-scale holes with the capability of controlling their geometries with precisions <10 µm and surface finishes with roughness <1 µm.

Evolutions of entrances and exits
To demonstrate the effect of the AOI control and the parameter compensation more clearly, the representative results machined under different conditions are summarized in figure 9 for comparison: (a) As discussed earlier, vertical sidewalls can be produced at an AOI of 0 • with the 5-axis scanner but accompanied by noticeably rough edges of large chamfers indicated by ring-shaped features at the entrances (figure 9(a)). (b) Through merely applying larger AOIs to minimize the impact of beam divergence, the edges at the entrances can be effectively reduced (figure 9(c)) but at a cost of increased sidewall tapers, which are indicated by two concentric circles observed from the exits. The larger and smaller circles represent the exit and the inner diameters of the entrances, respectively (figure 9(d)).
(c) The scanning-diameter compensation was first applied to simultaneously control both sidewalls and edges. The sidewall tapers resulting from larger AOIs are reduced, indicated by a narrower gap between the two circles representing the exit and the inner diameters of the entrances (figure 9(f)). However, the effect of scanning-diameter compensation saturates after reducing the gaps between the exit and the inner diameters of the entrances by around 40 µm and, thus, is not sufficient to completely eliminate the sidewall tapers. (d) Through applying both the scanning-diameter and laserpower compensations, the sidewall tapers are further reduced ( figure 9(h)). When the scanning-diameter compensation is fixed, the sidewall taper obviously decreases when more laser-power compensation is used and eventually becomes unnoticeable (figure 9(j)). Meanwhile, the entrances remain sharp and smooth (figures 9(g) and (i)), and the edge widths at the entrances remain below 10 µm. (e) As a comparison, excessive laser-power compensation causes smaller diameters at the exits than at the entrances, indicated by two smooth and concentric circles observed from the entrances. The larger and smaller circles represent the entrances and the exits, respectively (figure 9(k)).
Such evolution presents how the geometrical profiles and edge qualities are finely manipulated through AOI control and parameter compensation, providing an effective solution for machining macroscale holes requiring micro/nanoscale precisions. In addition to AOI which is most commonly used to tune tapers, this research indicates that controlling the fs laser ablation behaviors via parameter compensation can also realize tuning of sidewall tapers from positive to negative. Besides, it is worth noting that all experiments in this research were tested to show high repeatability.

Machining diverse and complex geometries
In addition to round holes, the AOI control and parameter compensation can also be used to machine complex geometries of macroscale sizes. For example, an elliptical hole (1 mm deep, 1 mm/500 µm wide in its major/minor axes) was machined using an AOI of −5 • , a diameter compensation of 400 µm, and a laser power of 5.6 W, the same as the optimized parameters for machining the round holes. No ring-like features on the entrance or concentric circles on the exit were observed (figure 10(a)), indicating that both a vertical sidewall and sharp edges were realized simultaneously. Furthermore, the same processing conditions were used to machine free-shaped geometries, e.g. a combination of two-crossed ellipses ( figure 10(b)) and a dumbbell-like geometry with two round holes and a bridge connecting them ( figure 10(c)). In spite of their complex shapes, vertical sidewalls along with sharp, clean entrances and exits were achieved on all parts of these geometries. The 3D profiling images collected by the laser scanning microscope further confirm their high-precision geometries with highquality edges and sidewalls. The success in machining such free-shaped geometries verifies that AOI control and the parameter-compensation strategy developed in this research has no obvious dependence on the geometries to be machined and, thus, can be used as a general approach to macroscale machining.

Conclusions
In this research, the key challenge in fs laser macroscale machining (i.e. achieving vertical sidewalls and sharp edges simultaneously) was addressed through developing an fs laser sharp shaping approach with controlled AOI and parameter compensation using a 5-axis scanner. The 5-axis scanner produced vertical sidewalls at an AOI of 0 • with obviously rough edges of large chamfers. By applying larger AOIs, the edges at the entrances were reduced, however, at a cost of increased sidewall tapers. Using AOI control and parameter compensation, we succeeded in achieving vertical sidewalls as well as sharp entrances and exits simultaneously for geometries of different shapes with both mm-scale dimensions and depths. The dimensional precisions were controlled below 10 µm while the surface finish was controlled below 1 µm in roughness. The laser-power compensation plays a dominating role in achieving non-tapered sidewalls when using larger AOIs. This research presents a novel strategy to finely control the fs laser machining process and establishes the capability of fs lasers in macroscale machining with micro/nanoscale precisions.