Fabrication of highly homogeneous and controllable nanogratings on silicon via chemical etching-assisted femtosecond laser modification

2.1 Fabrication of nanograting structures

The schematic of the experimental arrangement is demonstrated in Figure 1A. The femtosecond laser directly irradiated the sample surface focused by an achromatic doublet lens with a focal length of 100 mm. A Ti: sapphire regenerative oscillator-amplifier system (Spectra-Physics) was selected as the laser source, which provided a fundamental Gaussian laser beam with a central wavelength of 800 nm, pulse duration of 50 fs, and repetition rate of 1000 Hz. The laser fluence used in our experiments can be adjusted by controlling the half-wave plate and polarizer. Moreover, by manipulating the opening time of the mechanical shutter and the laser repetition rate, the number of pulses absorbed by the sample surface was tuned. Standard polished <100>Si wafers (10×10×1 mm) were pretreated with the HF solution to eliminate the surface oxide, followed by a 10-min ultrasonic cleaning. The processing parameters adopted in our experiments (except Figure S1) were as follows: laser fluence F=167 mJ cm⁻², laser repetition rate f=1000 Hz, and scanning speed v_scan=2000 μm s⁻¹. In the subsequent chemical etching procedure, the alkaline etchant was prepared by mixing 20 mass% KOH and isopropyl alcohol in a volume ratio of 4:1, and the ambient temperature was preserved at constant 55°C. All experiments were conducted in air.

Figure 1:

Schematic diagram of the experimental process.

(A) Schematic of the setup for femtosecond laser fabrication. (B) Flow chart of the chemical wet-etching assisted femtosecond laser modification approach.

2.2 Sample characterization

The morphology of the fabricated structures was examined with a scanning electron microscope (SEM, XL30S-FEG, FEI, Inc., Hillsboro, OR, USA). An atomic force microscope (AFM, Dimension Edge PSS, Bruker, Inc., Karlsruhe, Germany) was utilized to characterize detailed topographies of the nanogratings. The material crystallographic orientation properties in the laser irradiation area were measured by Micro-Raman spectroscopy (inVia reflex, Renishaw, Gloucestershire, London, UK) employing a 532 nm laser and transmission electron microscope (TEM, JEM 2100F, JEOL Ltd., Akishima, Tokyo, JAPAN). Structural colors ranging from red to purple were captured by an ordinary camera (Canon, EOS 6D, Tokyo, Japan) with varying viewing angles, which was performed under indoor light.

3.1 Processing and analysis of amorphous-silicon nanofringes

Ultrafast laser-induced melting and resolidification have been extensively studied to clarify the phase changes on silicon, that is, the amorphization of crystalline silicon and recrystallization of amorphous silicon [35], [36], [37], [38], [39]. In this study, we attempt to prepare amorphous silicon LIPSS (called amorphous-Si nanofringes in this paper) on crystalline silicon surface based on the SPPs surface wave excitation. As shown in Figure S1, bright nanofringes can be observed after statically irradiated by femtosecond laser pulses, using pulse number N=5, and laser fluence F=167 mJ cm⁻². The direction of the laser-induced amorphous-Si nanofringes is perpendicular to the polarization of incident laser and the average period of the nanofringes is approximately 790±30 nm, which correspond to the conventional low spatial frequency LIPSS. It is commonly known that polarized pulsed laser can induce sub-wavelength periodic surface structures on the material surface. However, the interaction between the laser and the material in the generation of LIPSS is a highly complex process. Thus, the formation mechanism still remains controversial. Even so, it is now widely accepted that the interference of the incident laser field with the excitation and propagation of SPPs plays a key role in the formation of LIPSS [12]. Theoretically, although SPP is generally merely coupled and propagated on metal surfaces, it can also occur in semiconductors and dielectrics in view of the fact that semiconductors or dielectrics will be transiently turned into a metal-like state upon the femtosecond laser irradiation [40].

The modified nanofringes on Si wafer can be extended to two-dimensional by converting laser static radiation into dynamic scanning, while the laser frequency and scanning speed should be carefully controlled [41]. Figure 2A illustrates the SEM micrograph of the 2D amorphous-Si nanofringes produced by femtosecond laser scanning through the sample with laser frequency f=1000 Hz and scanning speed v_scan=2000 μm s⁻¹ (the effective pulse number is calculated to be N_eff=3.8, as demonstrated in Figure S2), using the same laser fluence as Figure S1 (F=167 mJ cm⁻²). As presented in Figure 2A, the 2D alternating amorphous-crystalline nanofringes are regularly arranged on the Si wafer surface, and the black lines represent the amorphous-Si nanofringes. The period of the modified nanofringes is close to the wavelength of the incident laser (Λ=790±30 nm) [42]. The optical microscopy image of the sample is given in the upper right inset. Bright areas featuring increased reflectivity stand for laser exposed region, and the relatively dark areas featuring different reflectivity stay unexposed.

Figure 2:

Processing and analysis of amorphous-silicon nanofringes.

(A) SEM micrograph of the modified nanofringes on Si wafer produced by femtosecond laser scanning through the sample with a constant velocity v_scan=2000 μm s⁻¹, using laser fluence F=167 mJ cm⁻². (B) Raman spectra corresponding to the marked positions in (A). The red circle, blue triangle, and green square marked in (A) represent the position of amorphous-Si nanofringe, crystalline-Si nanofringe, and non-irradiation zones, respectively. The colors of the Raman spectra in (B) correspond to the marks in (A). (C) TEM image of the laser-modified region marked by a yellow circle in (A). (D) A partial enlarged view of (C).

We have measured the material crystallographic orientation characteristics of both the laser irradiated area and non-irradiated area through micro-Raman spectroscopy. Micro-Raman spectroscopy is a suitable tool for detecting the phase changing of silicon due to its high sensitivity to the crystallographic orientation of materials [12]. The red circle, blue triangle, and green square marked in Figure 2A represent the position of amorphous-Si nanofringe, crystalline-Si nanofringe, and non-irradiation zones, respectively. The resulting spectra of the corresponding detected positions in Figure 2A are illustrated in Figure 2B. As can be found in Figure 2B, the Raman spectrum of the non-irradiated area (green line) only presents the strong peak centered at 520.5 cm⁻¹ which is the characteristic peak of monocrystalline silicon. In contrast, the obtained spectrum of the laser-induced nanofringe (red line in Figure 2B) possesses a broad band centered at 473 cm⁻¹, which symbolizes the existence of amorphous silicon. Since the detecting depth of the micro-Raman spectroscopy is much larger than the thickness of the modified region, the Raman signal of the monocrystalline silicon substrate can be efficiently collected. Hence, the Raman spectrum of the amorphous-Si nanofringes exhibits a strong peak at 520.5 cm⁻¹ as well. We have also taken the region between two amorphous-Si nanofringes (the position marked by the blue triangle in Figure 2A) into consideration. The consequent spectrum only shows the characteristic peak of monocrystalline silicon.

A TEM test has also been performed to investigate the changing in the material property of the laser irradiated region. Figure 2C and D show the cross-sectional TEM images of the laser-modified area before chemical etching. The test sample was obtained from the position marked by the yellow circle in Figure 2A. As demonstrated in Figure 2C, an obvious boundary between the laser-modified shallow layer and the monocrystalline silicon substrate can be observed. Moreover, the high-resolution TEM image in Figure 2D clearly illustrates the changing in the crystallographic orientation of the monocrystalline silicon after laser irradiation. Atoms are in disordered arrangement in the laser-modified layer (the lower left part of Figure 2D), which is the characteristic feature of amorphous silicon, while the laser-unexposed substrate portion remains highly ordered (the upper right part of Figure 2D), implying the intrinsic nature of monocrystalline silicon. As expected, the laser-induced structure shown in Figure 2A is indeed the alternating amorphous-crystalline silicon nanofringes. The layer of the laser-induced amorphous Si is about 45.4 nm thick in this paper (as illustrated in Figure 2C), which corresponds well to the fabricated results by Izawa et al. [43].

3.2 Fabrication of nanogratings with high homogeneity and controllability

Nonhomogeneity has always been one of the main factors that hinder the widespread use of LIPSS. In the processing of traditional LIPSS, a relatively high laser fluence is utilized to bring forth local ablation of the material aiming at forming the grating structures. Laser-induced material ablation is always accompanied by the production of plenty of debris and surface defects, which will critically affect the propagation of SPPs and the subsequent distribution of light fields, leading to the nonhomogeneity of the fabricated structures. As shown in Figure 3A, rough pits in silicon are observed near the edge, and a majority of debris and recast layers are attached to the grating structures fabricated in a conventional pathway. On the contrary, the nanogratings fabricated in our experiment are highly homogeneous, which is generated in the following two steps: (i) amorphous-crystalline Si nanofringes are generated by focused femtosecond laser scanning through the silicon wafer surface, adopting constant scanning speed v_scan=2000 μm s⁻¹ and laser fluence F=167 mJ cm⁻² that are the same as in Figure 2A; (ii) the laser-modified sample is then placed in a KOH solution for chemical etching during which the laser-induced amorphous-Si acts as a subtle mask to prepare the designed structures, because the etching rate of the amorphous-Si in KOH solution is much slower than that of the crystalline Si. As shown in Figure 3B, the nanogratings fabricated by our technology possess high uniformity, and each grating distributes independently with no visible bifurcations (in Figure 3B, the etching time was 60 s). In addition, energy-dispersive X-ray analyses are performed in the laser exposed area and the laser unexposed area. As illustrated in Figure S3, there are no traces of oxygen in both regions, thereby eliminating the effect of the presence of surface oxygen on the etched topography. Hence, the regular arrangement of the laser-induced amorphous-Si nanofringes and the further chemical treatment would contribute to the great homogeneity. There are many factors affecting the etching rate of silicon in the alkaline solution (KOH), such as ambient temperature, solution concentration, etc., while the etching rate is also critically related to the dangling bond density and energy level of silicon [44], [45]. Under the same condition of constant concentration of alkaline solution and ambient temperature, the etching rate of (100) crystalline Si is nearly 30 times higher than that of amorphous Si, which is mainly attributed to the higher average dangling bond density and back bond energy level of the silicon atoms on crystalline Si [45], [46], [47], [48]. During the chemical etching, the (100) crystalline Si exposed to etching solution is quickly removed, while the laser-induced amorphous-Si nanofringes gradually protrude to form the nanogratings.

Figure 3:

Homogeneity comparison of the fabricated structures.

(A) SEM micrograph of the conventional LIPSSs fabricated by using laser scanning speed v_scan=1000 μm s⁻¹ and laser fluence F=450 mJ cm⁻². (B) SEM micrograph of nanograting structures fabricated by chemical etching assisted femtosecond laser modification, using scanning speed v_scan=2000 μm s⁻¹, laser fluence F=167 mJ cm⁻², and etching time T_et=60 s. (C) and (D) AFM images show the details of the surface morphology of (A) and (B).

To completely characterize the morphology of the traditional LIPSS and the nanograting structures fabricated by chemical etching-assisted femtosecond laser modification, we performed AFM analysis on both structures. The measured results are demonstrated in Figure 3C (traditional LIPSS) and Figure 3D (nanograting structures), respectively. Obviously, the nanograting structures fabricated by our technology (Figure 3D) are regularly distributed, closely aligned, and highly homogeneous compared with traditional LIPSS (Figure 3C).

However, the formation of regularly arranged nanogratings is critically sensitive to laser parameters. As demonstrated in Figure 4B, a weakly ablated area appears at the center of the nanofringes, while the effective pulses number is slightly increased from N_eff=3.8 (Figure 4A, using v_scan=2000 μm s⁻¹) to N_eff=5.1 (Figure 4B, using v_scan=1500 μm s⁻¹). Besides, when the effective pulses number is reduced to N_eff=1.9 (Figure 4C, using v_scan=4000 μm s⁻¹), only several separate modified points are obtained. The laser fluence adopted in the above three cases is constant (F=167 mJ cm⁻²). In addition to the effective pulses number, laser fluence also has dramatic influence on the amorphous nanofringes formation. In Figure 4D, as the laser fluence is set to 210 mJ cm⁻² (slightly higher than in Figure 4A) and the scanning speed is fixed at 2000 μm s⁻¹, a micro-scale modified line is generated in the laser-irradiated region rather than mutually independent nanofringes. The corresponding etched structures are illustrated in the SEM micrographs in Figure 4A–D. It’s clearly observed that only ordered laser-modified nanofringes, prepared at certain laser parameters, can be transformed into highly uniform nanograting structures during the subsequent chemical etching process. Notably, as demonstrated in Figure S4, large-area continuous nanogratings are successfully fabricated by precisely selecting the laser scanning interval (7 μm in our experiments) when applying our technology.

Figure 4:

Morphology dependence of nanogratings on femtosecond laser parameters.

The optical images in (A–D) are the laser-induced regions based on different laser parameters. The SEM micrographs in (A–D) are the corresponding etched results with etching duration of 90 s in KOH solution. The scales used in (A–D) are 3 μm.

Morphology controllability is a crucial characteristic of nanofabrication technique. Thus, in the following section, we are about to demonstrate the controllability of the nanogratings fabricated by chemical etching-assisted femtosecond laser modification approach. Namely, varying the chemical etching time enables flexible tuning of the height of the nanograting structures. Figure 5A–D show the SEM micrographs (left) and 3D AFM images (right) of different acquired nanograting structures while laser fluence and scanning rate remained constant (F=167 mJ cm⁻² and v_scan=2000 μm s⁻¹), but various etching time of 60 s, 90 s, 120 s, and 180 s were set, respectively. It is worthy to note that the heights of the structures increase by degrees from Figure 5A–D and are significantly higher than the laser-induced amorphous-Si nanofringes (Figure 2A). We measured the average heights and averages widths of the nanogratings demonstrated in Figure 5A–D. The statistical data in Figure 5E lead us to the conclusion that the average height of the nanogratings varies almost linearly over the etching time (the average heights in Figure 5A–D are as follows: 92±5 nm, 130±5 nm, 191±5 nm, and 256±5 nm). The inserts are the corresponding cross-section images of these four kinds of nanogratings, which clearly illustrate the height and shape of the gratings. That is to say that the height of the gratings can be precisely controlled by imposing limitations on the duration of the etching process. The length of the nanograting structure depends on the length of the laser-induced amorphous-Si nanofringe. Similar to the elapse of etching time, the average width of the grating structures undergoes a slight decrease. As shown in the right column of Figure 5E, the mean width values are 7.2±0.5 μm, 7.1±0.5 μm, 7.0±0.5 μm, and 6.7±0.5 μm, respectively).

Figure 5:

Morphology controllability of the nanograting structures.

(A–D) SEM micrographs (left) and AFM images (right) of the nanogratings by different etching processes (etching for 60 s, 90 s, 120 s, and 180 s, respectively). (E) Topography dependence of silicon nanograting structures on etching time. The inserts are the corresponding cross-section images.

3.3 Structural color display

Periodic surface structures which possess the quality of behaving as diffractive grating to generate structural color present a promising application prospect in the field of sample-based surface coloring. Similar to butterfly wings, when light irradiates the large-area nanograting structures, structural color can be clearly observed [49]. According to Figure 6B, diverse colors throughout the visible spectrum are obtained via varying the incident light angle and viewing angle. This appearance is summarized as the diffraction grating equation shown in Equation (1):

Figure 6:

“BIT”-shaped large-area nanograting structures fabricated by chemical etching-assisted femtosecond laser modification, using F=167 mJ cm⁻², v_scan=2000 μm s⁻¹, and T_et=90 s, respectively.

(A) SEM micrographs of the fabricated structure. (A <ii>) and (A <iii>) are the magnified depictions of (A <i>). The scales in (A) are 1 mm, 200 μm, and 5 μm, respectively. (B) The structural colors of the Si surface with 2D nanograting structures. Gradual changing of the viewing angle leads to the appearance of color red to purple shade due to the chromatic dispersion. (C) The structural colors of the Si surface adopting three different processing methods. A detailed comparison of the structural colors is shown in Videos S1–S3.

where m is the diffraction order, d is the period of the nanogratings (in this paper, d=790nm), and θ_I and θ_D represent the incident light angle and the diffraction angle, respectively [50]. Figure 6A shows the large “BIT”-shaped nanograting structures fabricated by the aforementioned method wherein chemical etching was employed for 90 s. Enlarged views of Figure 6A<i> are given in Figure 6A<ii> and <iii>. Figure 6B demonstrates the iridescence of Figure 6A captured by the camera when illuminating the structures with a beam of sunlight source. Keeping the angle of incident light unaltered, the light of different wavelengths is diffracted from corresponding diffraction angles. It is apparent from Figure 6B<i–vi> that while the viewing angle is gradually changed, various colors from red to purple are spotted.

Besides, the comparison of the structural colors of the surface structures generated by three different processing methods is performed in Figure 6C. Figure 6C<i> illustrates the fabrication of femtosecond laser-modified amorphous-crystalline structures without the subsequent etching process. The nanograting structures of Figure 6C<ii> were produced by conventional LIPSS method. Figure 6C<iii> pictures the nanograting structures fabricated by the approach mentioned above, noting that the chemical etching lasted for 90 s. The nanograting structures created by chemical etching-assisted femtosecond laser modification exhibit more favorable structural color characteristics, which is consistent with the spectroscopy measurements in Figure S5. This is attributed to the high homogeneity and controllable height of the nanograting structures induced by our method.

Moreover, the structural colors of the Si surface with complex patterned design are depicted in Figure 7, which are made up of 2D nanograting structures with cross scales from 790 nm to 0.75 cm. It is noteworthy that these 2D nanograting structures were fabricated by chemical etching-assisted femtosecond laser modification with etching for 90 s. This design is composed of the Chinese abbreviation and English full text of “Beijing Institute of Technology”. Figure 7A–C have displayed three different kinds of structural color viewing from several angles only under indoor light illumination (as shown in Video S4). These well-defined patterns will undoubtedly confirm that our approach is capable of producing any desirable pattern with remarkable structural color characteristics.

Figure 7:

A Si surface with complex patterned graphics made up of 2D nanograting structures with cross scales from 790 nm to 0.75 cm.

(A–C) The structural colors of the Si surface are observed from different angles captured by an ordinary camera under indoor illumination.