Periodicity control of laser-induced periodic nanostructures by thin deposition layer on sapphire substrate

Periodic nanostructures formed by ultrashort laser pulses are categorized as laser-induced periodic surface structures (LIPSSs). The remarkable characteristic of LIPSS is that the period of LIPSS is shorter than the applied laser wavelength. The LIPSS causes less damage to processed samples and formed by laser irradiation without mask and resist processes. Because of these advantages, LIPSS has attracted ever-increasing interest as a new fine processing technology. To establish the technology, understanding the formation mechanism, observation of the crystal state of the processing material, and control of desired structures are required.

Some formation mechanisms, such as surface plasmon polariton (SPP) excitation,^1–3) the parametric decay process,⁴⁾ and second-harmonic generation,^5,6) have been proposed. Further details are actively under discussion. Research regarding the crystal state of LIPSS still continues,^7–10) and we have previously reported that it depends on the irradiated material.⁷⁾ Previous published literature shows that the period of LIPSS changes depending on a number of factors including, the number of superimposed laser pulses,^11,12) laser energy fluence,^4,13,14) laser scanning rate,¹⁵⁾ laser wavelength,^14,16–19) and irradiated material.²⁰⁾ As these results showed, the period of LIPSS can be controlled by laser parameters, mainly the laser wavelength; however, the range and controllability obtained through this method are not enough for the desired control level.

In this study, we propose a new approach to control the period of LIPSS. It is shown that SPP, which is a major contributor to the formation of the LIPSS, is related to the dielectric constant of the irradiated material.^16,17,21) We attempt to control the period of LIPSS by depositing different kinds of material onto the processing material.

The applied source is a femtosecond laser oscillator (μ jewel D-10K, IMRA America Inc.; wavelength (λ_L): 1045 nm, pulse width: 450 fs, repetition frequency: 1 MHz). A sapphire substrate was used as the irradiated material. Sapphire has been widely used in industrial applications such as a ground substrate for light emitting diodes. This is due to the fact that sapphire has high hardness and excellent optical, chemical, and electrical properties while it is relatively inexpensive. The surface crystallographic orientation of the sapphire substrate was (0001), and the substrate surface was single-side polished by chemical-mechanical polishing. The parameter of the laser irradiation such as power were decided based on our previous research which formed LIPSS on sapphire substrates.²²⁾ As for the deposition layers with different dielectric constants, for the purpose of this research we chose Pt-Pd and ZnO to be deposited onto the sapphire substrate. Pt-Pd was deposited by magnetron cathode spattering (SC-701C-MC, Sanyu Electron),²³⁾ and ZnO by radio-frequency magnetron spattering (L-250S-FH, ANELVA)²⁴⁾ accordingly. Laser irradiation was performed onto the sapphire substrate both with and without deposition layer. In the case of irradiation onto sapphire with deposition layer, substrate was cleaned using acetone, hydrofluoric acid, ultra-pure water, and isopropyl alcohol for 10 min each after irradiation to remove the deposition layer and debris. The thickness of the deposition layer (D) has been set to vary between 5–2000 nm and the irradiated laser power was varied between 1–7 W (which corresponds to 0.6–4.2 J cm⁻²) to investigate their effects. The surface of the irradiated sample was observed by scanning electron microscopy (SEM: JSM-7800F, JEOL).

Surface SEM images of laser-irradiated substrate were shown in Fig. 1. The results for the laser irradiation on the sapphire substrate without deposition layer is shown in Fig. 1(a). LIPSSs were formed at higher than 4.5 W of laser power and no LIPSS was observed for lower than 4 W. The LIPSS was formed perpendicular to the direction of the laser polarization, which is represented as E in the figure. The period of the LIPSS (Λ) was approximately 330 nm. A structure distribution between the center and the above-and-below edges caused V-shaped ablation groove, and LIPSS was formed on the bottom or sidewall of the V-shaped groove. Such a profile comes from the energy distribution according to the Gaussian beam of the laser, and the scanning direction of the laser irradiation parallelly to the E.

Zoom In Zoom Out Reset image size

Figure 1(b) presents the results for the case of laser irradiation onto 2000 nm thick Pt-Pd layer or ZnO layer on the sapphire substrates. The irradiated laser power was 3.0 W. The left images show the LIPSSs just after laser irradiation and before removal of the deposition layer, and the right images show the LIPSSs after removal of the deposition layer (sapphire surface). In the case of the Pt-Pd layer, the period of the LIPSS was 160 nm and 140 nm before and after removal the Pt-Pd layer. In the case of the ZnO layer, the LIPSSs exhibit two kinds of periods simultaneously in the irradiated area, 120 nm and 280 nm before removal the ZnO layer and 110 nm and 280 nm after removal the ZnO layer. The corresponding periods after removal were slightly shorter than before removal. According to previous research on LIPSS formation on sapphire substrates by other groups,^5,25,26) the smallest period was 260 nm using a 800 nm laser wavelength. By applying 1045 nm laser wavelength we have achieved much smaller periods for LIPSS resulting from the deposition layer.

SEM images of the sapphire surface after removal the deposition layers in cases of laser irradiation through Pt-Pd or ZnO layers are presented in Fig. 2. The effect of the thickness of the deposition layer and the irradiated laser power are investigated. As it can be seen from the figure, the laser power threshold for LIPSS formation decreased due to the deposition layer in comparison to the case of sapphire substrate without the deposition layer that was mentioned above. The background color of SEM image depicts the classification of the period of LIPSS (Λ); yellow: Λ < 200 nm, red: 200 nm < Λ < 330 nm, white: no LIPSS formed. For every LIPSS formed through the deposition layer, the period was shorter than 330 nm, which is the period in the case directly formed onto the sapphire substrate. Some conditions showed that the LIPSS has two kinds of periods in the irradiated area. We believe that it comes from the energy distribution, as some other groups have also reported dependence of the period of LIPSS on the laser energy.⁴⁾ The results show that the LIPSS with shorter periods are formed at thicker deposition layers and lower laser powers. For some conditions, such as 1 W irradiation power on the 30 nm-Pt-Pd layer, some nanostructures kinds of precursor to the LIPSS were formed.^1,11)

Zoom In Zoom Out Reset image size

The period obtained for each condition is depicted in Fig. 3 while the horizontal axis represents the laser power. The periods tend to increase with the laser power in both kinds of periods (100–150 nm and 200–330 nm). In some conditions, LIPSS with the period of around 50 nm were formed, which is shorter than a twentieth of the laser wavelength. The period obtained for each condition is presented in Fig. 4 where the horizontal axis represents the thickness of the deposition layer (D). The majority of cases with thicker deposition layers than 40 nm exhibit the formation of LIPSS with shorter than 150 nm period, especially in the case of ZnO some other factors such as energy distribution would also affect this process and vary the results slightly. The periods both shorter than 150 nm and 200–330 nm did not show any large dependence on the thickness of the deposition layer.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Here, we discuss the mechanism of the periodicity change considering these results. The SPP wavelength related to the LIPSS period has been reported to have dependency on the dielectric constant of the irradiated material.^{1,3,16,17,21,27)} Miyaji et al. has reported that the change of the SPP wavelength caused by the SPPs excitation at the interface by the investigation of LIPSS formation on Si substrate immersed in water.³⁾ Similarly, our results also can be explained by the change of the SPP wavelength. The schematic model is presented in Fig. 5. The wavelength of the laser changes to λ_L' due to the change of the refractive index caused by the deposition layer when the thickness of the deposition layer D was thick enough. These manipulated light pulses afterwards excite the SPP at the interface between the deposition layer and sapphire substrate. The period of LIPSS (Λ_LIPSS) has been calculated using the following form:²¹⁾

$$\begin{eqnarray}&&{{\rm{\Lambda }}}_{LIPSS}=\displaystyle \frac{{{\rm{\Lambda }}}_{SPP}}{2}=\displaystyle \frac{{\lambda }_{L}}{\mathrm{Re}\left\{\sqrt{\tfrac{{\varepsilon }_{1}{\varepsilon }_{2}}{{\varepsilon }_{1}+{\varepsilon }_{2}}}\right\}}\end{eqnarray} \tag{ 1 }$$

where Λ_SPP is SPP wavelength, and ε₁ and ε₂ are dielectric constants. The exact measurement of the dielectric constant of the applied deposition layers in this research were not possible due to the lack of crystallinity. Therefore, we calculated the dielectric constant of Pt (ε_Pt) using the values from the literature.^23,28–32) The estimated period calculated to be 50–80 nm using ε_Pt and ε_Pt*, which is the dielectric constant of excited state of Pt, at the free electron concentration between 1E + 21–1E + 24 cm⁻³. The experimentally obtained values for the period are well-matched with the numerically calculated periods, though the details would be discussed using exact value of the dielectric constant of these deposition layers in future. In the case of thinner D, the period of the LIPSS mainly depended solely on the sapphire. This can be caused when the deposition layer would have negligible effect on the excitation of the SPP. In some case with higher laser power, the formation of V-shaped groove exhibited depths that were larger than the thickness of deposition layer. Such a feature also plays an important role in the formation of LIPSS with the period of approximately 330 nm. The effect of the deposited layer on the LIPSS formation process has been previously reported by other groups.^33–37) These studies presented the effect of the decrease of the energy fluence threshold for formation of LIPSS, and X. C. Wang et al.^36,37) additionally showed period shortening of the LIPSS on GaN/sapphire substrate compared with sapphire substrate. Though the detailed mechanism has not been explained, we are thinking the results obtained in these literatures and our report indicate that the interaction between different kinds of material would affect the LIPSS formation.

Zoom In Zoom Out Reset image size

In summary, we attempt to control the period of LIPSS for establishment of new fine processing technology. We demonstrated that the period of LIPSS was controllable by deposition layer of different kind of material on the sample. When laser irradiation was performed directly to the sapphire substrate, the period of the LIPSS was approximately 330 nm. In contrast, when the Pt-Pd or ZnO layers were deposited on the sapphire substrate, the period of the LIPSS formed at the sapphire surface was shortened. In the case of a thicker than 40 nm deposition layer, LIPSS with a much shorter period than 330 nm were formed while some even exhibited the period of around 50 nm. Even though further investigation on the detailed mechanisms involved and ways to improve the controllability are necessary, we proposed an active technique for controlling the period of LIPSS.

Acknowledgments