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Abstract
This paper presents an experimental study on the laser-induced atomic and close-to-atomic scale (ACS) structure of 4H-SiC using a capillary-discharged extreme ultraviolet (EUV) pulse of 46.9 nm wavelength. The modification mechanism at the ACS is investigated through molecular dynamics (MD) simulations. The irradiated surface is measured via scanning electron microscopy and atomic force microscopy. The possible changes in the crystalline structure are investigated using Raman spectroscopy and scanning transmission electron microscopy. The results show that the stripe-like structure is formed due to the uneven energy distribution of a beam. The laser-induced periodic surface structure at the ACS is first presented. The detected periodic surface structures with a peak-to-peak height of only 0.4 nm show periods of 190, 380, and 760 nm, which are approximately 4, 8, and 16 times the wavelength. In addition, no lattice damage is detected in the laser-affected zone. The study shows that the EUV pulse is a potential approach for the ACS manufacturing of semiconductors.
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1. Introduction
Silicon carbide (SiC) is a reprehensive third-generation semiconductor, which has been widely used in biomedical, photonic, and quantum devices due to its excellent physical and chemical properties [1]. However, the fabrication of hard and brittle materials with wide bandgaps has always been a problem due to the relatively low processing precision (at the level of tens to hundreds of nanometers) by traditional processing methods. The surface finish of pulsed laser technology can be down to the nanometric scale, but further exploration of the sub-nanometric level is still a challenge [2–6]. Processing at the atomic and close-to-atomic scale (ACS), directly generates an atomic-scale surface structure on a particular material due to the removal or recombination of atoms. This is of great significance for the fabrication of atomically thin optical devices and quantum devices. Therefore, this study is devoted to investigating a new method for the processing of sub-nanometric structures on SiC surfaces using extreme ultraviolet (EUV) laser pulses.
EUV laser is an attractive research topic because of its high photon energy and importance for various scientific and technical applications. Owing to its short wavelength (λ), small absorption depth, and thermal diffusion length, EUV laser shows great advantages to the upcoming atomic and close-to-atomic scale manufacturing (ACSM) [7–10]. Recently, EUV irradiation on various materials has been reported, including metals [11], organic polymers [12], inorganic non-metallic materials [13], and semiconductors [14–17]. For instance, Zhang et al. reported the damage thresholds of 1.29 and 1.37 J/cm2 on CaF2 and Au irradiated by a 13.5 nm EUV pulse, and the damage of CaF2 was attributed to thermoelastic stress and re-solidification [11]. Ritucci et al. studied the ablation of CaF2 and LiF by a 46.9 nm EUV pulse [14]. Thresholds of 0.06 and 0.1 J/cm2 and effective absorption depths of 14 and 20 nm were obtained from the fitting of experimental data, and the formation of microcracks on an LiF surface was attributed to strong thermoelastic stress. For EUV irradiation on silica, the material removal by a 12.5 nm EUV pulse was attributed to Coulomb repulsion [13]. Moreover, the ablation of Si by EUV and infrared pulsed laser was compared [18], and the results show that EUV-generated plasma undergoes planar expansion in a narrow angular range instead of the spherical expansion under infrared laser irradiation, implying the importance of photoionization induced directly by EUV laser. Laser-induced periodic surface structures (LIPSSs) have also been detected on an EUV-irradiated surface [12,15,16]. The LIPSS shows a period (Λ) much larger than the wavelength of the EUV laser, which is different from that induced by a long-wavelength pulse. For a 46.9 nm EUV irradiation on BaF2 [15,16], the formation of an LIPSS with 400 nm spacing was attributed to the thermoelastic effect. In addition, the quantum ablation effect was regarded as the main reason for the LIPSS formation on a polymethylmethacrylate surface [12]. Although there are many studies on the interaction between EUV and matter, they focused on the scale from several to tens of nanometers. Hence, there is still a lack of knowledge at atomic and close-to-atomic scale.
Molecular dynamics (MD) simulation is a widely used method for studying the processes at ACS [19–21]. MD has been coupled with the two-temperature model (TTM) to study the EUV–matter interaction [22], which allows the calculation of the energy evolution of electronic and atomic subsystems. Using the MD–TTM method, EUV irradiation on many materials (e.g., silicon, diamond, amorphous carbon, fullerene, and gallium arsenide) has been studied to reveal the electron–lattice energy exchange, thermal and nonthermal phase transitions, Coulomb explosion, and ablation [23]. A novel MD–TTM model concerning the photoionization effect has also been developed to study EUV irradiation on Si at ACS, where the possibility of single atomic layer removal and high surface integrity was predicted [24].
In this study, a capillary-discharged EUV laser was used to explore the processing of 4H-SiC at ACS. Scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, and scanning transmission electron microscopy (STEM) were performed to investigate surface and subsurface characteristics. The formation of a sub-nanometric structure was examined through an MD–TTM simulation. The results show that EUV has the potential to process at ACS.
2. Methods
2.1. Experiment
The schematic view of the experimental setup is depicted in Fig. 1. The radiation source is a 46.9 nm EUV laser produced via capillary discharge provided by Harbin Institute of Technology. The pulse duration is 1.7 ns, and the single-pulse energy is 100 µJ. The laser beam was focused through grazing incidence using a toroidal mirror with a 195 mm focal length. An incident angle of 83° was applied. The detailed parameters of the laser and the spot were reported in [15–17,25]. A two-dimensional (2D) translation stage was employed to adjust the relative position. The target is a 4H-SiC (0001) face with a crystal structure. The original surface was measured via AFM within a size of 5 × 5 µm2 for three times, obtaining an initial surface roughness of less than 0.2 nm in Sa (arithmetical mean height). The target was irradiated by 1, 2, 3, and 25 EUV pulses. The laser was manually operated in the single-shot excitation mode with a pulse interval of approximately 1 min.
The topographies of the original and irradiated target surfaces were investigated via SEM and AFM operated at the tapping mode. The possible changes in the lattice structure and chemical composition of the irradiated target were explored through Raman spectroscopy at 532 nm, energy-dispersive X-ray spectroscopy (EDS), and STEM.
2.2. Simulation
MD–TTM simulation was performed to investigate the modification of 4H-SiC using the LAMMPS package [26], and the result was visualized using OVITO [27]. The schematic diagram of the model is shown in Fig. 2. The simulation box consists of a vacuum layer and a 4H-SiC bulk with 280000 atoms. A temperature layer of 300 K was set at the bottom using the Nosé–Hoover thermostat with a time relaxation constant of 0.1 ps. Two layers of atoms at the bottom, including 1400 atoms, were fixed. A simulation was conducted in a microcanonical (NVE) ensemble, and the time step was set as 0.5 fs. Because of the time and space limitations of the MD method, the pulse width and energy absorption depth were scaled to 1/7000 of that in the experiment and 100 nm, respectively. The results show that the scaling influences the threshold fluence value, but it has little effect on the evolution of the lattice structure near the threshold [24].
Fig. 2. Schematic diagram of the MD–TTM model and the crystal structure of 4H-SiC with a stacking sequence of ABCB.
The coupling of MD with the TTM was realized by Eqs. (1) and (2) [28]. Equation (1) presents the heat transfer in the electronic subsystem, where Te andTa are the electronic and atomic temperatures, respectively. The electronic specific heat (Ce) is 2.5 × 105 Jm–1K–1, and the electron thermal conductivity (ke) is set as zero, assuming that the material acts as a semiconductor [29]. The coupling between the electronic and atomic subsystems is described by the electron–phonon coupling coefficient G, which is 9.8 × 1018 Wm–3K–1 [30], and S represents the laser source term. The fluence was set to 100 mJ/cm2, corresponding to that in the experiment as discussed below. Coarse-grained cells were employed to solve the electronic temperature equation [31]. The number of cells in the x, y, and z directions are 6, 6, and 100, respectively, ensuring that each cell contains enough atoms to calculate the lattice temperature according to Eq. (2), where ${m_i}$ and ${v_i}$ are the mass and velocity of atom i, respectively, ${v_c}$ is the velocity of the mass center of a cell, ${k_b}$ is the Boltzmann constant, and ${N_{cell}}$ is the number of atoms in a cell. The interaction between atoms is described using Tersoff potential [32].
3. Results and discussion
3.1. Surface topography characterization
The targets were irradiated by up to 25 shots of EUV laser. However, no surface change was observed by the optical microscope, so SEM was performed to characterize the laser-affected zone. Figure 3 shows the SEM images of SiC after EUV irradiation with 1, 2, 3, and 25 shots. The laser-affected zone showed stripe-like structures that became obvious as the number of pulses increases, as shown in Fig. 3(a–d). The shape of the laser-affected zone was formed by the annular profile of the capillary-discharged laser beam due to the refraction of the laser beam in the gain medium, which has been discussed in previous reports [15,16]. The size of the laser-affected zone detected via SEM increased with the number of pulses, implying the accumulation effect where the target surface can be changed by the superposition of pulses at below-threshold fluences. In addition, the pulse energy is 100 µJ, corresponding to the fluence in the order of 100 mJ/cm2 (using the estimated size of 1 × 0.1 mm2). For single-pulse irradiation, the SEM profile showed only a slight difference from the original surface. Compared with the AFM measurements discussed below, it may be appropriate to assume that the threshold is below the single-shot laser energy used in the experiment.
Fig. 3. SEM images of 4H-SiC after EUV irradiation with different pulse numbers. (a) 1 shot. (b) 2 shots. (c) 3 shots. (d) 25 shots. The two solid yellow circles mark the location of the AFM measurements in Fig. 3.
The stripe-like structures in the laser-affected zone were caused by the uneven energy distribution of the beam and were identified via AFM. The intensity of surface modification is expressed by peak-to-valley height, which is the difference in height between an adjacent peak and valley in a stripe-like structure. The detected peak-to-valley height is less than 3 nm and shows less obvious change with pulse numbers. Figure 4 illustrates the AFM profiles at 1 and 25 shots. The corresponding measuring areas are marked in Fig. 2 with yellow solid circles. The height profile of the cross section in Fig. 4(a) shows a depth of approximately 0.4 nm, which corresponds to about two C–Si bilayers. Figure. 4(b) shows the height profile at 25 shots with a large size of 50 × 50 µm2. The surface feature is consistent with that in the SEM measurement. The maximum peak-to-valley height is approximately 2 nm. The detected stripe-like structure with minimum peak-to-valley height in laser-affected zone is similar with that under 1 shot laser irradiation. Considering that the size of the AFM measurement is one to two orders of magnitude smaller than that of the laser-affected zone, it was difficult to simultaneously measure the irradiated and unirradiated areas. Therefore, whether it is material removal at the valleys or rearrangement of surface atoms cannot be determined, implying that the formation of the stripe-like structures cannot be determined only from surface morphology.
Fig. 4. AFM measurements of the stripe-like structures induced by EUV irradiation. (a) 1 shot. (b) 25 shots.
3.2. Periodic surface structure at ACS
Another surface characteristic induced by EUV irradiation is the LIPSS. The periodic sub-nanometric structure, which has not been observed by SEM, was detected via AFM in the laser-affected zone except for the single-shot irradiation (the data is shown in Table 1). The height profiles of the periodic nanostructure induced by 25-shot irradiation show a structure characteristic within 2 × 2 µm2, as shown in Fig. 5(a). The corresponding 3D image is plotted in Fig. 5(b). The cross-sectional profile (Fig. 5(c)) along the white dash line marked in Fig. 5(a) gives a period of ∼190 nm and peak-to-valley height of ∼0.4 nm, which is in two C–Si bilayers. The period under the 2-shot irradiation (Fig. 5(d)) is ∼760 nm, which is four times that of the 25-shot irradiation. The 3D profiles of the periodic nanostructures for the 2-shot irradiation are presented in Fig. 5(e). The results show that the sub-nanometric structure becomes obvious as the pulse number increases. However, the surface roughness measured via AFM within 5 × 5 µm2 is 0.130 and 0.104 nm in Sa for the 2-shot and 25-shot irradiations, respectively, which is similar to that of the original surface. In particular, the period in the low-fluence area of the 25-shot irradiation is 760 or 380 nm instead of 190 nm, implying that the short-period structure is likely to be generated at a high energy. The periodic sub-nanometric structures were detected in the edge of the laser-affected zone and also the stripe-like structure region, as shown in Fig. 5(f) and Fig. 5(g) for the 3-shot and 25-shot irradiation, respectively. This periodic sub-nanometric structure shows a huge potential in light field manipulation and metasurface application at ACS [33].
Fig. 5. AFM profiles of the periodic nanostructure. (a–c) 25 shots. (d–e) 2 shots. (f–g) Periodic nanostructures in the stripe-like structure regions for 3 and 25 shots.
Table 1. Periodic surface structure parameters
Several explanations about the formation of an LIPSS are available. In the Sipe–Drude theory, the formation is attributed to the interference between the incident wave and surface electromagnetic wave scattered at a rough surface [34]. The scattering model was used to explain the formation of near-subwavelength ripple (0.4 < Λ/λ < 1). Another explanation is the surface plasmon polariton (SPP) theory, that is a revision of the scattering model, and the effect of surface electromagnetic wave (surface plasmons) was taken into account. The SPP theory based on a high-pulse fluence that can generate a high-excited surface shows that the coupling of incident waves and SPP generates an LIPSS [35]. The first type is usually explained by the two theories, which are based on a polarized laser and give a period similar to or smaller than the incident wavelength. For example, near-subwavelength ripples with period between 0.6λ and 0.85λ were generated on SiC surface using different pulse number of 266 nm laser [35]. However, the LIPSS in this work shows a period much larger than the wavelength (46.9 nm). The second type is attributed to the melting and re-solidification effect [12]. In the study of 1060 nm laser ablation on Ge, the formation of LIPSS was attributed to the re-solidification of thin molten strips on the solid substrate at low fluences, and the freezing of laser-induced capillary waves at high fluences [36]. The mechanism is not suitable here since no melting was observed in the current experiment, which was also confirmed in the simulation as discussed below. For EUV-ablated polymethylmethacrylate [12], the LIPSS generated by a capillary-discharged 46.9 nm laser was attributed to the quantum ablation effects. The periodic structure shows a period of 2.8 µm and a peak-to-peak height of 5–10 nm. In the study on femtosecond laser processing of SiC by Wu et al. [1], periodic nanostructures with periods from approximately 0.2 to 5.0 µm were detected via SEM, the formation of which was explained by a laser-induced crystal cleavage.
In this experiment, the periodic nanostructures were distributed evenly on stripe-like nanostructures, as shown in Fig. 5(f–g). The period is 4–16 times the wavelength, and the peak-to-valley height is only at the C–Si bilayer scale. Based on the experiment, a speculation was made for the reduction of the nanostructure period. The structure may be generated after the first two EUV pulses due to the interference effect of the incident beam passing through the toroidal mirror. The formation origins from the rearrangement of surface atoms caused by the laser-induced thermal effect, which is discussed in Sect. 3.4. It is then broken by subsequent pulses, which reduces the period and peak-to-valley height and results in a smoother surface. The process is similar to the formation of mastoid nanostructures in the 46.9 nm laser-affected zone on Ba2F [16].
3.3. Lattice structure characterization
Raman spectrum and EDS measurement were applied to investigate the lattice structure in the laser-affected zone. The Raman spectroscopy under 25 EUV pulses as shown in Supplement 1 Fig. S1(a) presents a sharp peak at ∼775 cm–1 (transverse optic mode phonons), a peak at ∼203 cm–1 (transverse acoustic mode phonons), and a peak at ∼980 cm–1 (longitudinal optic mode phonons) ascribed to crystalline 4H-SiC. The Raman profile of the original surface is consistent with previously reported results [37]. It presents evidence that there is no obvious difference between the structural properties of the irradiated and original surfaces. In addition, the EDS curves in Supplement 1 Fig. S1(b) show a slight reduction in the Si element content, which, however, is within the error range and is not sufficient to deduce that there are changes in the surface elemental contents.
STEM measurement was conducted to further reveal the formation mechanism of stripe-like nanostructures and the lattice structure in the subsurface. The STEM image in Fig. 6 shows a lattice constant c of 1.04 nm by calculating the distance among 25 C–Si bilayers, which is in good agreement with the value (1.05 nm) reported previously [38] and corresponds to a 0.26 nm spacing between C–Si atomic bilayers. In addition, no subsurface damage was detected in the STEM measurement. In the MD study of 46.9 nm irradiation on Si [24], photoionization was assumed to be the main reason for the material removal at ACS, and only one to two atomic layers could be removed from the bulk with almost no lattice damage. Therefore, material removal is expected to occur during the irradiation process, and the influence of EUV on the surface could be confined to only a few atomic layers without damaging the lattice structure.
3.4. Modification mechanism at the atomic-layer scale
The surface modification at ACS was investigated through an MD simulation. The spatial–time evolution of the temperature field of the atomic subsystem is plotted in Fig. 7. The temperature rapidly rose during the laser energy deposition. The surface temperature reached 3100 K, which is close to the melting point of 3103 K [39]. During the first 50 ps, the thermodynamic processes (including electron–phonon coupling and heat transfer from the surface to the interior) take place, in which the electron–phonon coupling dominates. Then, the overall temperature of the simulation system decreases in the cooling stage, in which the thermal conduction of the atomic subsystem dominates.
The snapshots of the MD simulation are presented in Fig. 8. The expansion and contraction of the lattice are observed in Fig. 8(a). No structure damage was detected in the subsurface region, which is consistent with the experimental result. The irradiated surface is approximately 0.8 nm above the initial surface at the end of the simulation, which is attributed to the thermal expansion due to the relatively high system temperature (approximately 900 K). Although the system was not cooled to room temperature, the lattice structure showed almost no change in the last few picoseconds, implying that the system has evolved into a stable state. The surface modifications marked in Fig. 8(a) with a solid orange circle are presented in Fig. 8(b), which mainly appears in the first C–Si atomic bilayer, as shown in the inserted picture. The height of the modification is approximately 0.2 nm, which is consistent with the experimental result shown in Fig. 4(a). The corresponding dynamics processes include the break of the C–Si bond and the binding of new C–Si, C–C, and Si–Si bonds, leaving the final modified region. The modification mainly occurs along the two equivalent cleavage directions of [$11\bar{2}0$] and [$\bar{1}2\bar{1}0$].
Fig. 8. Snapshots of the MD simulation. (a) Structure topology as a function of time. (b) Enlarged view of the surface modification area circled in (a).
According to the experimental investigation of ACS structures and the MD simulation, the mechanism of the structures induced by the EUV pulse is established. Under the irradiation of EUV pulses, several C–Si bilayers are modified with the break of C–Si bonds and the recombination of C–C, C–Si, and Si–Si. This process leads to the formation of ACS structures. Under the irradiation of multiple pulses, the lattice was further split in the middle of the periodic structure. The thermal-induced rearrangement of surface atoms at ACS promotes the formation of periodic structures with small periods and peak-to-valley heights.
4. Conclusion
An EUV irradiation experiment on 4H-SiC (0001) Si-face was conducted using a capillary-discharged 46.9 nm laser with a pulse number ranging from 1 to 25, and the modification mechanism was investigated through an MD–TTM simulation. The experiment shows stripe-like nanostructures with a peak-to-valley height of less than 3 nm due to the uneven laser energy distribution, and the new periodic ACS surface structures have periods of about 4, 8, or 16 times the wavelength and peak-to-valley height at the scale of two C–Si bilayers. The mechanism is expected to be related to the broken and recombination of C–Si bonds based on the simulation, which illustrates the modification at the atomic-layer scale. In addition, no lattice structure damages were detected, implying the huge advantage of EUV pulses on surface processing at ACS.
Funding
National Natural Science Foundation of China (52035009, 62005066).
Acknowledgments
Thanks to Shan Wu, Yan Xu, and Huimin Qi for the discussions.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available from the corresponding author upon a reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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