Investigation of the Damage Characteristics and Mechanisms in Silicon Carbide Crystals Induced by Nanosecond Pulsed Lasers at the Fundamental Frequency

Abstract

Silicon carbide (SiC) single crystals are extensively utilized in various fields due to their exceptional properties, such as a wide bandgap and a high breakdown threshold. Nevertheless, the intrinsic high hardness of SiC creates significant challenges for contact machining. This study investigates the surface damage characteristics and underlying mechanisms involved in processing both high-purity silicon carbide (HP-SiC) and nitrogen-doped silicon carbide (N-SiC) crystals using fundamental-frequency nanosecond pulsed lasers. This study establishes a laser-induced damage threshold (LIDT) testing platform and employs the internationally standardized 1-ON-1 test method to evaluate the damage characteristics of HP-SiC and N-SiC crystals under single-pulse laser irradiation. Experimental results indicate that N-SiC crystals exhibit superior absorption characteristics and a lower LIDT compared with HP-SiC crystals. Subsequently, a defect analysis model was established to conduct a theoretical examination of defect information across various types of SiC. Under fundamental-frequency nanosecond pulsed laser irradiation, N-SiC crystals demonstrate a lower average damage threshold and a broader defect damage threshold distribution than their HP-SiC counterparts. By integrating multi-dimensional analytical methods—including photothermal weak absorption mechanisms and damage morphology analysis—the underlying damage mechanisms of the distinct SiC forms were comprehensively elucidated. Moreover, although N-SiC crystals show weaker photothermal absorption properties, they exhibit more pronounced absorption and damage response processes. These factors collectively account for the different laser damage resistances observed in the two types of silicon carbide crystals, implying that distinct processing methodologies should be employed for nanosecond pulsed laser treatment of different SiC crystals. This paper elucidates the damage characteristics of various SiC materials induced by near-infrared nanosecond pulsed lasers and explores their underlying physical mechanisms. Additionally, it provides reliable data and a comprehensive mechanistic explanation for the efficient removal of these materials in practical applications.

1. Introduction

Silicon carbide (SiC) crystals, recognized as being among the third-generation semiconductor materials, exhibit exceptional properties such as a wide bandgap, high thermal conductivity, and a high breakdown threshold [1,2,3,4,5,6,7]. These attributes render them indispensable for applications spanning aerospace, new energy vehicles, controlled fusion diagnostics, and various sectors within the electronic and information industries [8,9,10,11,12,13,14]. SiC single crystals are increasingly utilized across diverse fields, driving a growing demand for efficient and precise subtractive machining techniques such as cutting and drilling. However, owing to their exceptional hardness and brittle nature—with hardness second only to diamond—these crystals present significant challenges for precision machining. Pulsed lasers exhibit characteristics such as short pulse durations and high-power densities. Unlike traditional contact machining methods, such as diamond wire cutting, pulsed laser processing offers advantages in efficiency, precision, and minimized material loss. Consequently, these attributes make pulsed laser machining a highly promising approach for material removal in the processing of SiC crystals.

There have been numerous reports on the non-contact precision machining of SiC crystal using pulsed lasers [15,16,17,18,19,20,21,22]. Wu et al. employed femtosecond pulsed lasers to investigate the removal mechanisms of SiC crystal in various environments, including air, water, and hydrofluoric acid solutions [20]. Their findings demonstrate that the processing environment significantly influences the laser ablation threshold. Tomita et al. investigated the formation of micro- and nanostructures on 4H-SiC crystal surfaces via ultrashort laser pulse irradiation. They found that introducing artificially prepared scratches with specific orientations effectively reduced the incident laser energy density required to generate high-spatial-frequency structures, while exerting minimal influence on the threshold for inducing low-spatial-frequency features [22]. Previous studies have primarily focused on investigating the characteristics and mechanisms underlying the ultra-short pulse laser processing of SiC crystals. In contrast, a systematic exploration of the damage characteristics and mechanisms induced by nanosecond pulse lasers in these crystals remains to be conducted. Compared with femtosecond and picosecond pulse lasers, nanosecond pulse lasers introduce thermal effects during processing, which preclude cold processing conditions and consequently reduce machining precision. Nevertheless, nanosecond pulse lasers can achieve higher power outputs, presenting significant potential for applications involving large-scale material removal in SiC crystals. Therefore, further research into the associated material damage mechanisms is urgently needed. This study diverges from previous investigations on the mechanisms of ultra-short pulse laser precision machining of silicon carbide by addressing the efficiency and cost challenges associated with high-efficiency cutting of large-diameter silicon carbide crystals for industrial applications. It analyzes the material behavior during nanosecond pulse laser processing, demonstrating its suitability for these challenging scenarios. Moreover, the study examines the damage characteristics and mechanisms induced by laser irradiation in silicon carbide crystals using a multidimensional approach that incorporates weak photothermal absorption analysis, defect modeling, and damage morphology assessment. These findings provide comprehensive theoretical and experimental support for advancing the efficient nanosecond pulsed laser processing of silicon carbide crystals in future industrial applications.

This study investigates the damage characteristics and underlying mechanisms in high-purity silicon carbide (HP-SiC) and nitrogen-doped silicon carbide (N-SiC) crystals induced by fundamental-frequency nanosecond pulsed lasers. Initially, using the 1-ON-1 laser-induced damage threshold (LIDT) testing method, we measured the LIDT on the surfaces of HP-SiC and N-SiC crystals. Subsequently, by incorporating defect analysis models, we obtained critical defect information—including defect density, average damage threshold, and damage threshold distribution—for the various types of SiC crystal. Meanwhile, a photothermal weak absorption testing platform was developed to characterize the absorption properties of fundamental-frequency laser pulses on various SiC crystal surfaces. This platform facilitated the analysis of absorption characteristics associated with distinct crystal defects and verified the defect information obtained from a defect analysis model. Furthermore, differences in damage morphologies between HP-SiC and N-SiC were examined to elucidate the underlying damage mechanisms. This study systematically investigates the physical mechanisms responsible for damage induced by fundamental-frequency nanosecond pulsed lasers on SiC crystal surfaces and provides critical data to support the precise control of SiC crystal processing using these lasers.

2. Experimental Details

2.1. Test Sample

In this study, HP-SiC and N-SiC crystals were utilized as subjects. To minimize the influence of processing factors when comparing the damage characteristics and mechanisms of these two SiC types, both crystal surfaces underwent precision polishing. This process achieved a surface roughness of less than 0.1 nm and a Total Thickness Variation (TTV) below 1.6 μm, thereby meeting wafer-grade standards. Due to the distinct properties and functional applications of the two SiC crystals (as shown in

Table 1. The physical parameter of HP-SiC and N-SiC crystals.

Parameters	HP-SiC	N-SiC
Crystal structure	4H	4H
Crystal surface	$\left(000\overline{1}\right)$ C-plane	$\left(000\overline{1}\right)$ C-plane
Total Thickness Variation (TTV)	<1.6 μm	<1.6 μm
Density (25 °C)	3.21 g/cm3	3.21 g/cm3
Resistivity	1010 Ω·cm	22 mΩ·cm
Near-field spot size	20 μm	20 μm

), HP-SiC—with its exceptional thermal conductivity, hardness, and stability—serves as an ideal material for lens substrates and crystal heat sinks. Consequently, it is processed into circular samples ($\Phi 17 \mathrm{m}\mathrm{m}\times 1 \mathrm{m}\mathrm{m}$) using water-guided laser cutting. In contrast, conductive N-SiC, primarily utilized in semiconductor chip manufacturing, is processed into square samples ($10 \mathrm{m}\mathrm{m}\times 10 \mathrm{m}\mathrm{m}\times 1 \mathrm{m}\mathrm{m}$) via grinding wheel machining. Finally, to objectively assess the nanosecond laser-induced damage characteristics of various SiC crystals, all laser irradiation experiments were conducted on the C-plane of both HP-SiC and N-SiC [23].

2.2. Laser-Induced Damage Testing Platform

This study establishes a damage threshold testing platform for fundamental-frequency nanosecond pulsed lasers, as depicted in Figure 1. The Nd:YAG pulsed laser employed in the investigation emits a nanosecond pulse at a central wavelength of 1064 nm, with a pulsed duration of 7.6 ns (full width at half maximum) and a near-field spot size of 6.8 mm (at 1/e) under S-polarization. After passing through a laser energy attenuation system composed of a half-wave plate and a polarizer, the laser beam is directed toward a beam splitter. It splits the beam into two distinct paths: one leading to an energy meter and the other to a beam quality analyzer. This configuration enables real-time monitoring of the energy levels and near-field spot characteristics during the LIDT testing process. The incident laser beam passes through the beam splitter in the transmission direction and is subsequently focused by a lens with a 300 mm focal length, ultimately irradiating the sample surface with a focal-spot area of 0.04 mm². A CCD camera equipped with a 20× magnification system is employed to monitor, in real time, any damage to the incident surface of the SiC crystal. Residual laser energy transmitted through the test sample is absorbed by an absorption baffle. The specific laser parameters for the LIDT test are shown in

Table 2. The specific laser parameters for the LIDT test.

LIDT Test	Optical Parameter
Wavelength	1064 nm
Duration	7.6 ns
Near-field spot size	6.8 mm
Polarization	S
Focused length	300 mm
Focal-spot area	0.04 mm2

.

The laser damage threshold test method used in this study is the 1-ON-1 test method, as specified in the international standard (ISO 21254) [24]. This method, also known as the single-pulse laser damage threshold test, involves irradiating each position on the test specimen with a single laser pulse; regardless of whether damage occurs, the irradiation then moves to the next area. A CCD camera is employed to record any irreversible structural changes on the specimen following laser irradiation, thereby determining whether the laser exposure induces material damage. Single pulsed laser irradiation was applied at multiple positions on the test samples using a constant incident laser energy density. The probability of laser-induced damage was quantified as the ratio of observed damage events to the total number of irradiation trials, yielding a damage probability. By recording these probabilities across a range of energy density levels and performing linear regression, the incident laser energy density corresponding to zero damage probability was determined. This value was then established as the laser damage threshold for the material.

To compare the nanosecond pulsed laser damage thresholds of the two materials, it is essential to carefully select the laser fluence steps and determine the number of irradiations per fluence step. Although the sample sizes of the HP-SiC and N-SiC crystals differed, a consistent approach was adopted by selecting twelve fluence steps for each crystal, with ten irradiations conducted at each step, thereby ensuring an objective assessment of their nanosecond laser damage resistance. This methodology not only enables an accurate comparison of the laser damage thresholds but also facilitates the acquisition of more precise defect information.

2.3. Photothermal Weak Absorption Test Apparatus

Photothermal weak absorption leverages the thermal effects induced in materials during the heating process from pump laser interaction, which in turn leads to measurable phase shifts in the probe light. These phase shifts allow for theoretical quantification of the material’s photothermal weak absorption response to the pump laser. In this study, a pump laser with a central wavelength of 1064 nm and an incident power of 2.5 W is employed. A chopper maintains the laser frequency at 200 Hz, while a 1064 nm half-wave plate in combination with a polarizer enables precise adjustment of the pump laser’s incident power. Subsequently, a focusing lens directs the pump laser onto the test sample surface, achieving a spot diameter of 20 μm. The probe light is generated by a He-Ne laser with a central wavelength of 632.8 nm. Precise adjustment of the incident laser power is achieved using an attenuator composed of a half-wave plate and a polarizer, both optimized for 632.8 nm. Through coordinated adjustments of mirrors and focusing lenses, the probe light spot is made comparable in size to the pump light spot, facilitating overlapping irradiation on the sample surface, as illustrated in Figure 2. The sample is translated in increments of 0.05 mm, with each test covering an area of 2.5 mm × 2.5 mm. By measuring the photothermal weak absorption characteristics at multiple positions on the sample, the absorption properties of various surface regions are determined. When the SiC crystal surface absorbs the pump laser, localized temperature increases occur. These thermal effects induce phase changes in the detection light, enabling the derivation of the corresponding photothermal weak absorption coefficient through theoretical calculations. The corresponding parameters for the photothermal weak absorption testing system are shown in

Table 3. The specific laser parameters for the photothermal weak absorption test.

Photothermal Weak Absorption Test	Pump Light	Probe Light
Wavelength	1064 nm	632.8 nm
Power	2.5 W	0.6 W
Duration	200 Hz	CW
Near-field spot size	20 μm	20 μm

.

Regarding the photothermal weak absorption testing, the relevant testing procedures were as follows: First, a pulsed pump laser, centered at a 1064 nm wavelength, is used. After passing through an attenuator consisting of a half-wave plate and a polarizer, the pump laser power is modulated. To ensure that sufficient power induces crystal defect absorption and generates the photothermal effect without causing material damage, the pump laser power was set at 2.5 W. Subsequently, the pump laser beam was directed through a chopper operating at 200 Hz, which transmits the modulated signal to a phase-locked amplifier. The beam was then focused by a lens, forming a 20 μm diameter focal spot on the sample surface. Finally, after traversing the sample, the laser power was measured in real-time using a power meter. Subsequently, continuous light emitted by a He-Ne laser with a central wavelength of 632.8 nm was used as the detection beam. After passing through an attenuator—comprising a half-wave plate and polarizer, both optimized for 632.8 nm—the detection beam power is adjusted to 0.6 W, ensuring that it does not interfere with the interaction between the pump light and crystal defects. The incidence direction of the detection light is then modified using a mirror, and a focusing lens concentrates the beam, maintaining identical positions and sizes for both the pump and detection beams on the test sample surface. The detection light reflected from the crystal surface passes through a filter and aperture before being captured by a photodetector. The signal was subsequently transmitted to a card reader and then processed by a phase-locked amplifier, which recorded the phase shift in the detection light induced by the thermo-optical effect during the interaction between the pump laser and the crystal. This measurement enables a theoretical calculation of the material’s photothermal weak absorption coefficient corresponding to the thermo-optical effect during the pump laser-SiC crystal interaction. It was worth noticing that the photothermal weak absorption test system described herein constitutes a comparative measurement method. Its principle is based on the generation of a thermal envelope effect when pump light (1064 nm) interacts with crystal defects. Subsequently, as probe light passes through the region exhibiting this thermal envelope, its phase is altered. By recording these phase shifts, one can theoretically calculate the relative weak absorption value at a given position in comparison to defect-free crystal regions. Unlike laser calorimetry, which provides absolute measurements, this method employs the photothermal weak absorption values of calibrated components as reference standards, rendering it only suitable for comparing the weak absorption characteristics of different crystals rather than determining precise photothermal weak absorption values.

3. Experimental Results

Table 4. Laser-induced damage thresholds of HP-SiC crystal and N-SiC crystal.

	HP-SiC (J/cm2)	N-SiC (J/cm2)
LIDT	12.04	8.06

presents the results of one-on-one laser damage threshold tests for HP-SiC and N-SiC crystals subjected to fundamental-frequency nanosecond pulsed lasers. The test standard employed in this study is the single-pulse laser damage threshold method (1-ON-1 LIDT test method), as defined by ISO 21254 [24]. This method entails irradiating a specimen with a single pulse of laser light at a designated location. Regardless of whether damage occurs, the laser is subsequently repositioned to a new location for further testing. By varying the incident laser fluence across a series of steps and recording the corresponding damage probabilities, the laser fluence at which no damage is observed is determined and designated as the LIDT. This LIDT serves as a quantitative measure of the material’s resistance to initial damage. The data indicate that HP-SiC crystals exhibit a higher laser damage threshold than N-SiC crystals. It was worth noting that during the LIDT test, identical laser fluence steps and laser irradiation shots were employed to compare the nanosecond pulsed laser damage thresholds at the fundamental frequency for various SiC crystals, thereby ensuring accurate comparative measurements. To mitigate the effects of disparate crystal growth processes on the evaluation of laser damage resistance, and to objectively analyze the damage mechanisms in different SiC crystals, five HP-SiC and five N-SiC crystals were selected from distinct batches. The presented results correspond to the experimental outcome closest to the average threshold value among the five tests, ensuring both experimental precision and statistical reliability. Additionally, for the photothermal weak absorption evaluation, five distinct locations on each of the five samples were tested, confirming the accuracy of the photothermal weak absorption test in these SiC crystals. This finding provides critical insights for the precision machining of both types of SiC crystals using fundamental-frequency nanosecond pulsed lasers. For instance, when employing these lasers for the precision cutting and wafering of large-diameter SiC crystals, it is essential to strictly control the incident laser energy density for N-SiC crystals near their damage threshold to minimize processing losses. In contrast, for HP-SiC crystals, increasing the incident laser energy density appropriately can enhance processing efficiency and productivity.

Meanwhile, the characteristics and mechanisms underlying multi-pulse cumulative irradiation of SiC crystals pose significant challenges for processing these materials using nanosecond pulsed lasers. This topic remains a critical focus of our ongoing research. In this study, we conducted a series of investigations addressing these challenges. Initially, using the S-ON-1 laser-induced damage threshold testing method (frequency = 5 Hz, S values = 5, 10, 30, 50), we determined the multi-pulse laser damage thresholds for two types of SiC crystals. The test results are summarized in

Table 5. The LIDTs induced in HP-SiC and N-SiC crystals under different pulse number irradiation at the same repetition rate (Frequency = 5 Hz).

	HP-SiC (J/cm2)	N-SiC (J/cm2)
S = 5	10.62	6.17
S = 10	8.89	4.36
S = 30	6.38	2.63
S = 50	5.01	1.02

. Experimental results indicate that under identical repetition rates and multi-pulse laser irradiation conditions, both HP-SiC and N-SiC crystals exhibited a reduction in LIDT, with the decrease becoming more pronounced as the number of pulses increased. Notably, the decline in multi-pulse LIDT was more significant for N-SiC crystals than for HP-SiC crystals. This observation suggests that, under multi-pulse laser irradiation, the cumulative effects enhance defect-mediated absorption in N-SiC crystals to a greater extent than in HP-SiC crystals. The more pronounced reduction in LIDT for N-SiC might be attributed to the diverse lattice defects introduced during its doping process. Consequently, when employing fundamental-frequency nanosecond pulsed lasers of identical fluence for multi-pulse irradiation of different SiC crystals, rigorous control over the number of pulses and repetition rate is essential for N-SiC crystals to prevent excessive ablation.

Moreover, variations in multi-pulse laser damage thresholds across diverse laser fluence gradients have also been examined. We employed the R-ON-1 laser damage threshold testing method, whereby each location is subjected to irradiation with progressively increasing energy densities in increments of 0.5 J/cm², as detailed in

Table 6. The LIDTs for both HP-SiC and N-SiC crystals were determined using the R-ON-1 laser damage threshold test method.

	HP-SiC (J/cm2)	N-SiC (J/cm2)
LIDT (R-ON-1)	14.65	12.81

. The results demonstrate that the R-ON-1 laser damage thresholds for both types of SiC crystals exceed their respective 1-ON-1 damage thresholds, indicating an effective laser conditioning effect. Notably, the threshold enhancement is more pronounced in N-SiC crystals compared to HP-SiC crystals, suggesting that the laser conditioning more effectively mitigates threshold-lowering lattice defects introduced during the doping process of N-SiC crystals. The underlying physical mechanism will be investigated in subsequent studies.

4. Discussion

4.1. Spectral Absorption Testing

To elucidate the physical mechanisms underlying the differences in laser damage thresholds observed in two types of SiC crystals, this paper examines four key aspects: spectral absorption, defect characterization, weak photothermal absorption properties, and damage morphology analysis. Firstly, this study conducted absorption spectroscopy measurements on two types of SiC crystals using a Cary 5000 spectrophotometer, Santa Clara, CA, USA (see Figure 3). The spectral range spanned from 200 nm to 1500 nm. Notably, at a wavelength of 1064 nm in the near-infrared region, HP-SiC exhibited a transmittance of 66%, whereas N-SiC showed only 18%. These findings indicate that N-SiC crystals possess significantly higher absorption, which is a primary factor contributing to their reduced damage threshold.

However, the damage threshold did not exhibit the substantial variation observed in the absorption spectrum. This observation indicates that the damage properties of SiC crystals induced by fundamental-frequency nanosecond lasers are not solely determined by the material’s spectral absorption characteristics; rather, additional physical mechanisms are involved. Subsequently, this paper presents separate analyses from the perspectives of weak photothermal absorption, defect characterization, and damage morphology and processes.

4.2. Defect Precursor Information

The mechanisms underlying laser-induced damage differ between nanosecond and ultrafast pulsed lasers, with material defect information playing a crucial role in determining damage characteristics. Silicon carbide crystals exhibit a diverse range of surface defects—including both intrinsic crystal flaws and those introduced during processing—which display varying capacities for laser absorption and corresponding resistance to laser damage. Moreover, these defects are randomly distributed across the crystal surface. Consequently, acquiring detailed information about these defects is essential for accurately analyzing the damage characteristics of HP-SiC and N-SiC crystals.

This study employs the statistical model proposed by Krol et al. [25,26], which assumes that defect damage thresholds follow a Gaussian distribution, in conjunction with the results of the 1-ON-1 laser damage threshold tests described above, to analyze defect characteristics in two types of SiC crystals. The model postulates that both the mean defect damage threshold (${{\rm T}}_0$) and the defect damage threshold distribution ($\Delta {\rm T}$) within the test specimen adhere to a normal distribution function. Consequently, the corresponding defect function can be expressed as:

$$g\left({\rm T}\right)={\displaystyle \frac{2d}{\Delta {\rm T}\sqrt{2\pi }}}\mathit{exp}\left[-{\displaystyle \frac{1}{2}}{\left({\displaystyle \frac{{\rm T}-{{\rm T}}_0}{\frac{\Delta {\rm T}}{2}}}\right)}^2\right]$$

(1)

$$\mathrm{d}={\int }_0^{\infty }g\left({\rm T}\right)d{\rm T}.$$

(2)

During damage testing, the total number of defects within a specified spot area is integrated to yield the defect density ($\mathrm{d}$). In 1-ON-1 laser damage threshold testing, this result represents the probability of damage corresponding to different incident laser energy densities. Consequently, it is necessary to combine the aforementioned defect analysis model with the Poisson distribution to establish a connection between the defect statistical model and the damage probability. Within the equivalent spot area, $S_{eff}=\pi {\omega }_x\times {\omega }_y/2$, the region where the laser energy density exceeds the threshold ($F>T$) is denoted as $S_T\left(F\right)$:

$$S_T\left(F\right)=S_{eff}\ln \left({\displaystyle \frac{F}{T}}\right).$$

(3)

$$N\left(F\right)={\int }_0^{F}g\left(T\right)S_T\left(F\right)dT.$$

(4)

In this equation, $N\left(F\right)$ represents the number of defects within the laser-illuminated region where the energy density ($F$) exceeds the defect threshold ($F$) necessary to induce damage. Consequently, the damage probability, denoted as $P\left(F\right)$, is expressed using the Poisson formula:

$$P\left(F\right)=1-exp\left[1-N\left(F\right)\right].$$

(5)

As a result, the defect information corresponding to HP-SiC and N-SiC crystals can be obtained, as illustrated in Figure 4 and detailed in

Table 7. The defect information of different SiC crystal.

	$\mathbf{d}$ (/mm2)	${\mathit{{\rm T}}}_{\mathbf{0}}$ (J/cm2)	$\mathbf{\Delta }\mathit{T}$ (J/cm2)
HP-SiC	7.19	12.52	1.44
N-SiC	4.63	9.8	3.74

. Notably, the N-SiC crystals exhibit a lower average damage threshold (${{\rm T}}_0$) than the HP-SiC crystals, indicating that under identical energy density absorption conditions, the surface defects in N-SiC crystals are more prone to laser-induced damage. This disparity is the primary factor leading to the difference in overall single-pulse laser damage thresholds between the two types of SiC crystals. Meanwhile, N-SiC crystals exhibit a broader distribution of defect damage thresholds ($\Delta {\rm T}$) relative to HP-SiC crystals, suggesting that they harbor a more diverse array of defects. These findings will be corroborated through subsequent analyses of their photothermal weak absorption characteristics.

It is noteworthy that N-SiC crystals exhibit a broader defect damage threshold range while maintaining a lower defect density compared to their HP-SiC counterparts. It was mainly caused by the combined influence of the LIDT test outcomes and defect analysis models. In the LIDT testing, the laser fluence for HP-SiC crystals spans approximately 12.5 J/cm² to 17.5 J/cm² over the 0% to 100% damage probability spectrum, whereas for N-SiC crystals, the range is approximately 7.5 J/cm² to 17 J/cm². This observation indicates that the defect damage energy density range for N-SiC crystals is broader than that for HP-SiC crystals. In the defect analysis model presented, defect data simulation involves fitting the defect information within the portion of the incident spot where the laser fluence exceeds the defect damage threshold. Consequently, for HP-SiC crystals, the narrower defect damage fluence range signifies that a smaller area of the incident spot experiences defects spanning the full damage probability spectrum. As a result, the defect fitting procedure yields a higher defect density for HP-SiC crystals compared to N-SiC crystals.

4.3. Photothermal Weak Absorption Characteristics

Nanosecond pulsed laser-induced material damage is primarily governed by thermal mechanisms arising from the defect absorption of laser radiation. The inherent weak photothermal absorption serves as a critical indicator for characterizing defect distribution and absorption capacity. This approach addresses the “thermal envelope effect” produced during pump laser–material interactions by employing theoretical calculations based on phase changes observed in the recorded probe light, thereby determining the spatial variations in photothermal absorption properties. The experimental setup is depicted in Figure 2.

In this study, to objectively compare the photothermal weak absorption characteristics of HP-SiC and N-SiC crystals, photothermal weak absorption tests were performed at five distinct positions on the surfaces of both SiC types. The test area measured 2.5 mm × 2.5 mm with a step length of 0.05 mm. The results, shown in Figure 5 and summarized in

Table 8. Photothermal weak absorption of HP-SiC crystal and N-SiC crystal.

	HP-SiC	N-SiC
Average value (ppm)	19.68	34.23
Maximum value (ppm)	148.63	230.17

, indicate that the average photothermal weak absorption value of the HP-SiC crystal (19.68 ppm) is lower than that of the N-SiC crystal (34.23 ppm). This observation suggests that surface defects in N-SiC crystals exhibit stronger laser absorption characteristics than those in HP-SiC crystals, which aligns with earlier findings indicating that N-SiC crystals have a lower average damage threshold (${{\rm T}}_0$). Moreover, the photothermal weak absorption dynamic range of N-SiC crystals, with a maximum value of 230.17 ppm, exceeds that of HP-SiC crystals, which reached 148.63 ppm. These results further corroborate the defect analysis, indicating that the defect damage threshold range ($\Delta T$) for N-SiC crystals is greater than that for HP-SiC crystals, thereby confirming the accuracy of the defect information obtained for each SiC crystal. Therefore, different SiC crystals exhibit distinct photothermal weak absorption characteristics, which are the primary factors responsible for variations in their damage thresholds.

4.4. Damage Morphology Analysis

The preceding discussion analyzed the mechanisms underlying the distinct damage characteristics of the two SiC types by examining spectral absorption, defect analysis, and photothermal weak absorption properties. In this section, we investigate the variations in damage behavior by analyzing the morphologies induced in different SiC crystals by fundamental-frequency nanosecond pulsed lasers. Both HP-SiC and N-SiC crystals were subjected to an identical incident laser energy density of 31.37 J/cm² to induce surface damage, and the resulting damage morphologies were documented using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometer (EDS), as illustrated in Figure 6.

Firstly, the damage morphology demonstrates that both HP-SiC and N-SiC crystals exhibit ablation under nanosecond pulsed laser irradiation, indicating that thermal effects remain the dominant damage mechanism in both types of SiC at the fundamental frequency. It was worth noticed that the laser damage testing platform employs a spot with an area of 0.04 mm², corresponding to a diameter of approximately 120 μm. Consequently, when comparing the damage morphology of different crystals, the damage dimensions of the N-SiC crystal was comparable to the spot size. Moreover, we exposed both SiC crystals to an identical incident laser fluence (31.37 J/cm²) to induce initial surface damage at five distinct positions. SEM was then employed to record and compare the dimensions and depths of the damage, with the results summarized in

Table 9. The initial damage size of HP-SiC crystal and N-SiC crystal under the same incident laser fluence irradiations.

	HP-SiC	N-SiC
Damage depth (Average value)	0.87 μm	0.89 μm
The lateral damage size (Average value)	82.36 μm	124.17 μm

. It could be observed that for the identical incident laser fluence, the damage depths of the two silicon carbide crystals remained essentially consistent, but the surface damage area of N-SiC is consistently larger than that of HP-SiC. This observation further corroborates the previously noted differences in photothermal absorption characteristics between the two materials, with N-SiC exhibiting higher photothermal absorption. Consequently, under equivalent irradiation conditions, N-SiC absorbs more laser energy, resulting in a larger ablation damage area.

Subsequently, Si:C ratios at five distinct damage sites across various SiC crystals were recorded using an energy spectrometer (see

Table 10. Ratio of C and Si elements at different damage locations in HP-SiC and N-SiC.

C:Si	HP-SiC	N-SiC
Central damage area	44.7:55.3	43.4:56.6
Damaged peripheral areas	42.5:57.5	41.4:58.6

). The results indicate that the Si:C ratios in the central damaged regions of both crystals are comparable to those in the undamaged areas. This phenomenon can be attributed to the Gaussian distribution of the incident laser energy used in the damage threshold testing, which concentrates higher energy at the center. Consequently, during the induction of damage in the SiC crystals, the interaction between surface carbon and atmospheric oxygen leads to the formation and subsequent loss of carbon dioxide, while the high-energy laser induces carbon agglomeration at the surface. As a result, no significant depletion of carbon occurs.

In the damaged edge regions, the ablation process has just begun. Carbon within the crystal reacts with atmospheric oxygen, leading to the significant loss of carbon via gaseous byproducts such as carbon dioxide. Analysis of the carbon-to-silicon (C:Si) element ratio in regions subjected to identical incident laser energy densities shows that the reduction in carbon content is markedly greater in the damage margins of N-SiC crystals than in those of HP-SiC crystals. Consequently, under equivalent irradiation conditions, N-SiC exhibits more severe ablation damage, with a higher proportion of its carbon combining with oxygen for removal. This reaction constitutes one of the primary factors underlying its reduced damage threshold.

5. Conclusions

This paper addresses the challenges of precision subtractive machining in hard and brittle SiC crystals. It examines the damage characteristics and mechanisms imparted by fundamental-frequency nanosecond pulsed lasers on both HP-SiC and N-SiC crystals. A testing platform for evaluating damage thresholds using fundamental-frequency nanosecond pulsed lasers was first established. Utilizing the internationally standardized 1-ON-1 laser damage threshold method, the single-pulse laser damage thresholds for both types of SiC crystals were determined. The results indicate that, when subjected to fundamental-frequency nanosecond pulsed lasers, N-SiC crystals exhibit a lower LIDT compared to the HP-SiC crystal. Subsequently, spectral absorption measurements were conducted to analyze the transmittance of two types of SiC crystals. The test results indicated that the N-SiC crystal exhibits higher absorption than HP-SiC, which partially accounts for the observed disparity in damage thresholds. In addition, theoretical calculations using a defect analysis model were performed on both crystals, revealing distinct defect characteristics that further contribute to the lower damage threshold of N-SiC crystal. A photothermal weak absorption platform was also established to characterize the subtle photothermal absorption properties of both SiC crystals, yielding corresponding absorption profiles. Finally, scanning electron microscopy and energy-dispersive spectroscopy were employed to examine damage morphologies, uncovering distinct damage characteristics and underlying physical processes. This research advances precision control techniques for efficient nanosecond pulsed laser ablation processing of various SiC crystals.