# Experimental Investigation on Ablation of 4H-SiC by Infrared Femtosecond Laser

^{*}

## Abstract

**:**

^{2}and 4.97 J/cm

^{2}, respectively. In the multiple-pulse scribing ablation for microgrooves, the ablation threshold dropped to 0.70 J/cm

^{2}due to the accumulation effect when the effective pulse number reached 720. The calculated average of the thermally stimulated ablation depth of 4H-SiC is 22.4 nm, which gradually decreased with the raising of the effective pulse number. For obtaining square trenches with precise and controllable depths and a smooth bottom in 4H-SiC, the effects of processing parameters on the material removal rate and surface roughness are discussed. The ablation rate per pulse is almost constant, even if the effective pulse number varies. The reduction of laser spot overlapping ratio in x direction has a greater weakening effect on the material removal rate than that in y direction. The precise amount of material removal can still be controlled, while modulating the surface roughness of the ablated features by changing the hatch rotation angle. This research will help to achieve controllable, accurate, and high-quality machining results in SiC ablation, using infrared femtosecond laser.

## 1. Introduction

^{2}and 2.5 × 10

^{3}cm

^{−1}, respectively [20]. Molian measured the ablation depth per pulse of 1–18 nm and the ablation threshold of 2 mJ/cm

^{2}from a picosecond laser with a central wavelength of 1552 nm on 3C-SiC [21]. When the repetition rate of a laser pulse is less than 250 kHz, the laser-ablated trenches and holes indicated clean features and non-thermal damage. Zhang et al. studied the impacts of technological parameters on grooves of 4H-SiC processed by a femtosecond laser with a central wavelength of 800 nm and a maximum repetition rate of 1 kHz [22]. The depth and width of the grooves increased with increasing power, pulse repetition rate, and decreasing scanning speed. The surface roughness within 0.05 µm of the groove is obtained by increasing number of repeat scans. Wang et al. investigated the effect of repeated irradiation of femtosecond laser with a central wavelength of 1030 nm at near-threshold fluence of 1.1 J/cm

^{2}on the surface morphologies induced in 4H-SiC surface [23]. The results show that the inoculation effect on the subsurface of 4H-SiC lead to different energy absorption accumulation, which was the reason for the discontinuity of the ablation zone. Iwatani et al. investigated laser drilling underwater on 4H-SiC by using a nanosecond pulsed infrared (1064 nm) laser [24]. When the laser fluence is less than 10 J/cm

^{2}and the water thickness is 1 mm, vias without debris, heat-affected zones and cracks and were obtained. Wu et al. used a femtosecond laser with a central wavelength of 800 nm to ablate 4H-SiC in air, water, and hydrofluoric acid (HF), respectively [25]. The highest ablation threshold (4.98 J/cm

^{2}) occurred in air when the pulse number was 50 and the lowest ablation threshold (0.53 J/cm

^{2}) came from HF when the pulse number was 300. Li et al. studied the effects of processing parameters, including laser power, scanning speed, and scanning times, on the depth, width, and roughness (Ra) of microgrooves fabricated by a femtosecond laser with a central wavelength of 1064 nm on 4H-SiC through a response surface methodology and artificial neural network method [26].

## 2. Materials and Methods

#### 2.1. Materials

^{2}regions of ≤100 nm is used in the experiments. The laser processing is carried out on the C surface of the substrate. The 4H-SiC sample is ultrasonically cleaned with acetone and absolute ethanol before and after laser ablation.

#### 2.2. Laser Processing

## 3. Results and Discussion

#### 3.1. Measurement of Single-Pulse Ablation Threshold

_{0}is the peak energy density of the laser beam; r is the distance from any point on the spot to the center of the beam; ω

_{0}is the beam waist radius of the Gaussian profile at the focus. The peak energy density F

_{0}can be stated as follows:

_{a}is the average laser power; f is the repetition rate of the laser. The laser ablation threshold and waist radius can be obtained through numerical fitting using Liu’s method as follows [27]:

_{th}is the ablation threshold. Substituting for the peak fluence from Equation (2):

_{m}and the structural transformation spot diameter D

_{s}) and the logarithm of average laser powers are fitted in Figure 3. The beam waist radius ω

_{0}can be calculated from the slope of the linear fitting line, and the estimated values are 15.91 μm and 16.19 μm, respectively. The single-pulse ablation thresholds for material modification (F

_{th_m}) and structural transformation (F

_{th_s}) are calculated from the intercepts with the abscissa axis as 2.35 J/cm

^{2}and 4.97 J/cm

^{2}, respectively. Their corresponding laser average power are 1.868 W and 4.096 W, respectively.

#### 3.2. Multiple-Pulse Scribing Ablation for Microgrooves

_{a}) under different effective pulse numbers are described in Figure 5a. According to the method in Section 3.1, the multiple-pulse ablation threshold versus different effective pulse numbers is illustrated in Figure 5b. The fitting curve of the experimental data of multiple-pulse ablation follows the laser pulse energy accumulation model. It can be expressed as [30]:

_{th}(N) represents the corresponding ablation threshold when the effective pulse number is N; F

_{th}(∞) is the saturated ablation threshold when the effective pulse number is infinite; F

_{th}(1) is the single-pulse ablation threshold; k is an incubation parameter (independent of N in first approximation) that describes the intensity of defect accumulation and the enhancement in photon absorption. The larger k is the fewer effective pulse number N are needed to obtain the constant saturated ablation threshold F

_{th}(∞), below which an infinite number of laser pulses would not damage the structure of 4H-SiC. It can be found that when the N is less than 240, the multiple-pulse ablation threshold decreases rapidly with the raising of the N. When the N reaches 720, the laser pulse energy accumulation effect reaches saturation and the ablation threshold is 0.70 J/cm

^{2}, which is a 70% reduction compared to the single-pulse ablation threshold for material modification (F

_{th_m}). When the ablation threshold is saturated, it can be considered that F

_{th}(∞) = F

_{th}(720) = 0.70 J/cm

^{2}. According to the experimental result of single-pulse ablation in Section 3.1, F

_{th}(1) = F

_{th_m}= 2.35 J/cm

^{2}. Thus, the calculated incubation parameter k is 0.0199. It is worth noting that the saturated ablation threshold F

_{th}(∞) measured by multiple-pulse experiment is less than the single-pulse ablation threshold for material modification (F

_{th_m}). This means that even if the laser fluence is lower than F

_{th_m}, as long as it is higher than the saturated ablation threshold F

_{th}(∞), the subsequent macroscopic damage of the material can be promoted when the N accumulates to a certain value. This conclusion can also be obtained from the ablation experiments of other materials [31].

_{R}), which is estimated through dividing the depth of the ablated microgroove by the effective pulse number [32]:

_{eff}is the effective absorption coefficient. Its reciprocal is defined as the thermally stimulated ablation depth, which includes the thermal diffusion length and the light penetration depth [21]:

_{0}/F

_{th}) for different effective pulse numbers is plotted in Figure 6b. According to Equation (7), the slope of each linear fitting function in Figure 6b represents the thermally stimulated ablation depth of 4H-SiC. The effective absorption coefficient α

_{eff}of 4H-SiC can be furtherly calculated. The relationship between the thermally stimulated ablation depth and the effective absorption coefficient and the effective pulse number is shown in Figure 6c. The arithmetical average of the thermally stimulated ablation depth of 4H-SiC is 22.4 nm, which is comparable to the result of (21 nm) [33]. There is evidence that the surface temperature of the material in the laser irradiation region would rise with the increase in the pulse number [34]. The α would increase with rising temperature due to the ionization of the dopant and thermal activation of the valence band electrons [35,36].

#### 3.3. Large-Area and Bulk Ablation

#### 3.3.1. Analysis of Process Parameters Effect on the Material Removal Rate

_{a}) is fixed at 2.169 W, and the repetition frequency (f) is fixed at 200 kHz, so different effective pulse numbers (N) are obtained by changing the scanning speed (v) and the number of scans (K) to process square trenches with dimensions of 1 × 1 mm

^{2}. The hatch style of laser scanning is to fill a series of parallel paths. The effect of hatch spacing is also considered. The parameter combinations of the experimental groups are shown in Table 2.

_{x}) and y (δ

_{y}) directions, as shown in Figure 7, are precisely controlled respectively through setting laser scanning speed (v) and hatch spacing (L) [37]:

_{R}) is almost constant even if the effective pulse number (N) varies. This is important for the machining requirements of large-area bulk ablation, and it means that a precise amount of material removal can be effectively controlled to achieve the desired depth of structure. Moreover, the ablation rate (A

_{R}) defined by the effective pulse number (N) is not affected by the scanning speed at the same hatch spacing (L). The effect of hatch spacing (L) on laser ablation behavior is significant. Figure 9 illustrates the bottom surface topography of deep trenches machined using different hatch spacings. As the hatch spacing increases from 5 μm to 15 μm, δ

_{y}decreases, that is, the effective ablation of the material in the overlapping area of the laser spot in the y direction weakens, and the ablation rate (A

_{R}) decreases. When the hatch spacing reaches 30 μm, δ

_{y}is 0%, and the microgrooves scribed by each scanning path are separated from each other, resulting in an ablation rate (A

_{R}) substantially equal to that of scribing ablation shown in Figure 6a under the same average laser power. Essentially, the ablation rate hardly decreases after δ

_{y}is below 50%.

_{e}):

_{R}) for L = 5, 10 and 15 μm in Figure 8 to the material removal rate (R

_{e}), and converted v and L to δ

_{x}and δ

_{y}, respectively. Error bars represent the deviation in calculated R

_{e}when machining the trenches through different times of scans. It can be found that the decrease of δ

_{x}and δ

_{y}will lead to the reduction of R

_{e}. When δ

_{x}and δ

_{y}decrease by the same amount, the contribution of δ

_{y}to the reduction of R

_{e}is greater than δ

_{x}.

_{x}and δ

_{y}on the R

_{e}at different P

_{a}of 1.687 W, 1.928 W, and 2.169 W are graphed together in Figure 11. It can be found that at different P

_{a}and δ

_{y}, R

_{e}drops gradually with the decrease of δ

_{x}. When δ

_{x}is lower than 50%, that is, the scanning speed exceeds 3000 mm/s, the weakening effect of δ

_{x}on R

_{e}is no longer obvious. The material removal behavior reaches an approximate saturation rate.

#### 3.3.2. Planarization Processing Surface

_{e}) is insensitive to changes in hatch rotation angle. This means that the precise amount of material removal can still be controlled while modulating the flatness of the ablated features.

## 4. Conclusions

^{2}and 4.97 J/cm

^{2}, respectively. When the effective pulse number reaches 720, the ablation threshold of 4H-SiC would drop to 0.70 J/cm

^{2}due to the laser pulse energy accumulation effect. The calculated average of the thermally stimulated ablation depth of 4H-SiC is 22.4 nm. The phenomenon that the absorption coefficient gradually decreases with the increase of the effective pulse number proves that heat accumulation still exists during the multiple-pulse ablation process. In large-area bulk ablation processing, the ablation rate per pulse is almost constant even if the effective pulse number varies. The reduction of laser spot overlapping ratio in x direction has a greater weakening effect on the material removal rate than that in y direction. The surface roughness is minimized at hatch rotation angles of 15° and 75° at different hatch spacings of 5, 10, and 15 μm. Changes in the hatch rotation angle do not result in variations in material removal rates.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 2.**Microscopic view of ablation spots on the 4H-SiC under different average laser powers form 2.169 W to 12.050 W.

**Figure 4.**LSCM images of ablated microgrooves (N = 240) at different average laser powers of (

**a1**) 1.687 W, (

**b1**) 1.928 W, (

**c1**) 2.169 W, (

**d1**) 2.410 W, (

**e1**) 2.651 W, and (

**f1**) 2.892 W and their profile data in (

**a2**–

**f2**). LSCM images of ablated microgrooves (N = 12) at different average laser powers of (

**g1**) 1.687 W, (

**h1**) 1.928 W, (

**i1**) 2.169 W, (

**j1**) 2.410 W, (

**k1**) 2.651 W, and (

**l1**) 2.892 W and their profile data in (

**g2**–

**l2**).

**Figure 5.**(

**a**) The widths of all the ablated microgrooves versus the average laser powers under different effective pulse numbers. (

**b**) The relationship between effective pulse number and ablation threshold.

**Figure 6.**(

**a**) The ablation rate of the microgrooves versus the average laser power. (

**b**) The ablation rate versus ln(F

_{0}/F

_{th}) for different effective pulse numbers. (

**c**) Results of the thermally stimulated ablation depth and the effective absorption coefficient for different effective pulse numbers.

**Figure 7.**Illustration of overlapping ratios of the laser spot in x (δ

_{x}) and y (δ

_{y}) directions.

**Figure 8.**The ablation rate (A

_{R}) of the trenches versus the effective pulse number (N) at different hatch spacings (L).

**Figure 9.**The bottom surface topography of deep trenches machined using different hatch spacings of (

**a1**) 5 μm, (

**b1**) 10 μm, (

**c1**) 15 μm and (

**d1**) 30 μm and their enlarged views of (

**a2**–

**d2**) at the average laser power of 2.169 W and the scanning speed of 1000 mm/s.

**Figure 10.**The material removal rate (R

_{e}) versus the overlapping ratios of the laser spot in x (δ

_{x}) and y (δ

_{y}) directions at the average laser power of 2.169 W.

**Figure 11.**The material removal rate (R

_{e}) versus the overlapping ratios of the laser spot at different average laser powers.

**Figure 12.**(

**a1**–

**g1**) SEM images and (

**a2**–

**g2**) LSCM measurements of the square trenches machined only by changing the hatch rotation angle (θ) from 0° to 90° at the laser average power of 2.169 W, the scanning speed of 1000 mm/s, and the hatch spacings of 5 μm.

**Figure 13.**The material removal rate (R

_{e}) and surface roughness (Sa) versus the hatch rotation angles at different hatch spacings of 5, 10, and 15 μm.

Model | Value |
---|---|

Central wavelength | 1035 nm |

Pulse width | 300 fs |

Maximum output power | 40 W |

Repetition rate | 25 kHz–5 MHz |

Power stability | <2% |

Pulse energy | 80 μJ |

Beam quality | M2 < 1.3 |

Beam divergence | <2 mrad |

Number | Scanning Speed (mm/s) | Number of Scans | Effective Pulse Number | Hatch Spacing (μm) |
---|---|---|---|---|

1 | 1000 | 10 | 60 | 5, 10, 15, 30 |

2 | 2000 | 20 | ||

3 | 3000 | 30 | ||

4 | 1000 | 80 | 480 | 5, 10, 15, 30 |

5 | 2000 | 160 | ||

6 | 3000 | 240 | ||

7 | 1000 | 200 | 1200 | 5, 10, 15, 30 |

8 | 2000 | 400 | ||

9 | 3000 | 600 | ||

10 | 1000 | 320 | 1920 | 5, 10, 15, 30 |

11 | 2000 | 640 | ||

12 | 3000 | 960 | ||

13 | 1000 | 1000 | 6000 | 5, 10, 15, 30 |

14 | 2000 | 2000 | ||

15 | 3000 | 3000 |

v (mm/s) | Δx | L (μm) | δy |
---|---|---|---|

1000 | 83% | 5 | 83% |

2000 | 67% | 10 | 67% |

3000 | 50% | 15 | 50% |

6000 | 0% | 30 | 0% |

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**MDPI and ACS Style**

Wang, L.; Zhao, Y.; Yang, Y.; Zhang, M.; Zhao, Y.
Experimental Investigation on Ablation of 4H-SiC by Infrared Femtosecond Laser. *Micromachines* **2022**, *13*, 1291.
https://doi.org/10.3390/mi13081291

**AMA Style**

Wang L, Zhao Y, Yang Y, Zhang M, Zhao Y.
Experimental Investigation on Ablation of 4H-SiC by Infrared Femtosecond Laser. *Micromachines*. 2022; 13(8):1291.
https://doi.org/10.3390/mi13081291

**Chicago/Turabian Style**

Wang, Lukang, You Zhao, Yu Yang, Manman Zhang, and Yulong Zhao.
2022. "Experimental Investigation on Ablation of 4H-SiC by Infrared Femtosecond Laser" *Micromachines* 13, no. 8: 1291.
https://doi.org/10.3390/mi13081291