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Abstract
We demonstrate sub-micrometer processing of two kinds of thin films, polymethyl methacrylate (PMMA) and metal nano-particle resist, by focusing high-order harmonics of near-IR femtosecond laser pulses in the extreme ultraviolet (XUV) wavelength region (27.2–34.3 nm) on the thin film samples using an ellipsoidal focusing mirror. The ablation threshold fluences for the PMMA sample and the metal nano-particle resist per XUV pulse obtained by the accumulation of 200 XUV pulses were determined to be ${0.42}\;{{\rm mJ/cm}^2}$ and ${0.17}\;{{\rm mJ/cm}^2}$, respectively. The diameters (FWHM) of a hole created by the ablation on the PMMA film at the focus were 0.67 µm and 0.44 µm along the horizontal direction and the vertical direction, respectively. The fluence dependence of the Raman microscope spectra of the processed holes on the PMMA sample showed that the chemical modification, in which ${\rm C} {=} {\rm C}$ double bonds are formed associated with the scission of the PMMA polymer chains, is achieved by the irradiation of the XUV pulses.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Processing and machining by light sources in the extreme ultraviolet (XUV) wavelength region have attracted attention over recent years because of their capabilities of sub-micrometer focusing with a long working distance in the millimeter range [1,2], facilitating micro processing and micro fabrication of solid materials. When we irradiate a solid sample with short-pulsed XUV light whose pulse duration is in the range of 10 ps–10 fs, we can efficiently transfer the photon energy to a sub-micrometer size area of the sample surface within the short period of time thanks to the high absorption coefficient of solid materials in the XUV wavelength range [3,4]. Therefore, if we adjust appropriately the energy of the XUV pulses, we can efficiently suppress thermal degradations in the peripheral zone around the processed area, resulting in the suppression of the formation of a heat-affected zone (HAZ) [5].
So far, in XUV light processing, plasma-based light sources and soft X-ray FEL sources have been used for the investigation of the ablation processes of solid materials [6–14], as well as for the mask projection in the micrometer and sub-micrometer sizes [15–17]. On the other hand, recent advances in the technology of high-order harmonics (HHs) generation using near-IR intense femtosecond laser pulses enabled us to develop a table-top femtosecond pulsed XUV laser sources. Even though the pulse energies of such HHs light sources are 2–3 orders of magnitude smaller than those of the FEL sources, the HHs light sources in the XUV wavelength are promising light sources for material processing [11,18–20]. Indeed, polymethyl methacrylate (PMMA) was processed by the HHs light pulses for the monitoring of a spatial profile of the HHs light pulses [11,18]. However, in these studies, the light fluences were around ${0.3}\;{{\rm mJ/cm}^2}$, which is in the evaporation regime of PMMA and is 1 order of magnitude smaller than the ablation threshold. Recently, the nano-focusing of the XUV light was demonstrated with reflective optics such as a Schwarzschild objective [21] and an ellipsoidal mirror [22]. The resulting fluence at the focus of the HHs light pulses can be raised to be in the ablation regime so that the sub-micrometer size processing of solid materials can be realized [22].
In this Letter, we perform processing of thin-film samples of PMMA and metal nano-particle resist at the sub-micrometer scale by irradiating the samples with focused femtosecond XUV light pulses generated as the HHs of near-IR intense femtosecond laser pulses. We create sub-micrometer scale holes on the PMMA sample and the metal nano-particle resist by irradiating the samples with multiple HHs pulses whose orders are in the range between the 23rd and the 29th corresponding to the wavelength range of 27.2–34.3 nm. We investigate the dependence of the depth of the holes created on the sample surface on the fluence of the XUV pulses to discuss the transition from the evaporation regime to the ablation regime. We also investigate the chemical modification of the PMMA sample by the irradiation of the XUV pulses by the measurements of Raman microscopy spectra of the hole areas.
We generated the HHs in the XUV wavelength region by focusing Ti:sapphire laser pulses (790 nm, 4 mJ, ${\sim} {40}\;{\rm fs}$, 1 kHz) in a semi-infinite gas cell [23] filled with an Ar gas. After filtering out the near-IR laser pulses using an Al filter (300 nm in thickness), we focused the XUV pulses by an ellipsoidal mirror [22]. The energies of the XUV pulses were recorded as the current output of a micro-channel plate (MCP) detector, which were calibrated with respect to the intensity of the images of the XUV light beam recorded using a CCD camera. From a total current integrated over 150, 300, and 500 shots of the XUV pulses calibrated by the CCD image intensities, the average number of photons and the pulse energy per single XUV pulse were determined to be ${8.0} \times {{10}^6}$ photons and 56.4 pJ/pulse, respectively, with the estimated errors of 20%.
The PMMA (Sigma-Aldrich, ${\rm M. W}. = {350}{,}{000})$ sample with a thickness of 250 nm was prepared by spin-coating on an Si wafer whose thickness is 525 µm. The metal nano-particle resist with a thickness of 80 nm made of zirconium oxide core coated by methacrylic acid (EIDEC Inc.) [24–26] was prepared by spin-coating on an Si wafer. The PMMA sample and the metal nano-particle resist were irradiated with the XUV pulses at normal incidence with the fluence between 0.03 and ${2.5}\;{{\rm mJ/cm}^2}$. The focal spot sizes of the XUV pulses were measured by a knife edge scan. The profiles of the focal spot along the horizontal direction parallel to the $x$ axis and along the vertical direction parallel to the $y$ axis are shown in Fig. 1(a) as a function of the spatial position on the beam propagation axis (the $z$ axis), whose origin ($z = {0}$) is located at the focal point. The profiles along the $x$ (horizontal) and $y$ (vertical) directions measured at $z = {14.6}\;{\unicode{x00B5}{\rm m}}$ are shown in Figs. 1(b) and 1(c), respectively. The focal spot size along the $x$ direction defined as the full width at half-maximum (FWHM) at $z = {14.6}\;{\unicode{x00B5}{\rm m}}$ is ${1.48} \pm {0.18}\;{\unicode{x00B5}{\rm m}}$ and that the FWHM along the $y$ direction is ${0.79} \pm {0.08}\;{\unicode{x00B5}{\rm m}}$. The $z$ dependence of the focal spot size along the $x$ direction and that along the $y$ direction were fitted by a Gaussian beam profile, and the radius along the $x$ direction, ${w_x}(z)$, and that along the $y$ direction, ${w_y}(z)$, were determined as a function of $z$. Using ${w_x}(z)$ and ${w_y}(z)$, the divergence angles were determined to be 0.036 rad along the $x$ direction and 0.050 rad along the $y$ direction, respectively. In the irradiation experiment described below, we varied the fluence at the sample surface, which is estimated using ${w_x}(z)$ and ${w_y}(z)$, by changing the sample position along the $z$ direction.
Fig. 1. (a) Profiles along the $x$ (horizontal) direction and the $y$ (vertical) direction of the XUV pulses as a function of the position along the $z$ direction. The solid lines represent the best-fit curves obtained using the $z$ dependence of a Gaussian beam profile. (b) and (c) The focal spot profiles at $z = {14.6}\;\unicode{x00B5}{\rm m}$ as a function of the position along the $x$ direction and the $y$ direction, respectively. The solid lines represent the best-fit Gaussian profiles.
After the irradiation of the XUV pulses, we recorded the surface images using an atomic force microscope (AFM, Park Systems NX10) and performed the analysis of the morphology. In Fig. 2, an AFM image of a typical hole created on the PMMA surface at $z = {14.6}\;{\unicode{x00B5}{\rm m}}$ is shown with the depth profiles along the $x$ (horizontal) and $y$ (vertical) directions as the cross sections of the hole. The sizes of the hole, defined as the FWHMs of the depth profiles, were 0.67 µm (horizontal) and 0.44 µm (vertical), respectively. The AFM image of the hole created by the XUV pulses has a close resemblance to the XUV beam profile shown in Figs. 1(b) and 1(c).
Fig. 2. AFM image of the depth profile of the hole created on the surface of the PMMA sample by the irradiation of the 200 XUV pulses at the fluence of ${2.48}\;{{\rm mJ/cm}^2}$ ($z = {14.6}\;\unicode{x00B5}{\rm m}$) with the profiles of the cross sections along the horizontal direction and the vertical direction.
Figure 3 shows the surface removal rate, defined as an increment of the depth of a hole per XUV laser shot, for (a) the PMMA and (b) the metal nano-particle resist as a function of the irradiation fluence in a logarithmic scale. We obtained the surface removal rate by a single XUV pulse by dividing the depth of the hole created by the irradiation of 200 XUV shots. The depth of a hole after the irradiation of 200 XUV pulses was obtained from the AFM images as described above. The fluence of the XUV pulses was obtained as an averaged value of the energies of the XUV pulses over 200 shots divided by the XUV beam spot area measured by the knife edge scan.
Fig. 3. Surface removal rates of the PMMA sample (a) and the metal nanoparticle resist (b) by the irradiation of the focused XUV pulses (27.2–34.3 nm) as a function of the single-shot fluence. In each frame, a dotted straight line represents a best-fit line represented by Eq. (1) for the data in the low fluence region (the evaporation regime) and a solid line represents a best-fit line for the data in the high fluence region (the ablation regime).
As can be seen in Fig. 3(a), the surface removal rate for the PMMA sample increases as the fluence increases and exhibits a linear slope below the fluence of ${0.8}\;{{\rm mJ/cm}^2}$, and the linear slope becomes steeper in the fluence range above ${0.8}\;{{\rm mJ/cm}^2}$. On the other hand, as shown in Fig. 3(b), the surface removal rate for the metal nano-particle resist exhibits a similar dependence on the fluence, but the change in the slope occurs at the lower fluence of ${0.3}\;{{\rm mJ/cm}^2}$. The changes in the slopes in Figs. 3(a) and 3(b) indicate that the transition from the evaporation regime in the lower fluence region to the ablation regime in the higher fluence region [11]. It has been known that the surface removal rate, $d$, can be fitted by the following equation [27]:
where ${1/}{\alpha _{{\rm eff}}}$ denotes the effective penetration depth represented using the effective absorption coefficient (${\alpha _{{\rm eff}}}$), and $F$ and ${F_{{\rm th}}}$ denote the fluence of the XUV laser pulse and the threshold fluence for evaporation or ablation, respectively.By fitting the data plotted in Figs. 3(a) and 3(b) in the ablation regime to Eq. (1), we determined the ablation threshold fluence and the effective penetration depth for the PMMA sample to be ${F_{{\rm th}}}({200}) = {0.42} \pm {0.20}\;{{\rm mJ/cm}^2}$ and $({1/}{\alpha _{{\rm eff}}}) = {0.27} \pm {0.12}\;{\rm nm}$ and those for the metal nano-particle resist to be ${F_{{\rm th}}}({200}) = {0.17} \pm {0.07}\;{{\rm mJ/cm}^2}$ and $({1/}{\alpha _{{\rm eff}}})) = {0.045} \pm {0.016}\;{\rm nm}$, where the number in the parentheses for ${F_{{\rm th}}}$ represents the number of shots $N$. We also performed the irradiation of the PMMA sample with the XUV pulses and determined the multiple-shot ablation threshold fluences for 500, 1000, and 2000 XUV shots to be ${F_{{\rm th}}}({500}) = {0.27} \pm {0.12}\;{{\rm mJ/cm}^2}$, ${F_{{\rm th}}}({1000}) = {0.11} \pm {0.10}\;{{\rm mJ/cm}^2}$, and ${F_{{\rm th}}}({2000}) = {0.10} \pm {0.06}\;{{\rm mJ/cm}^2}$, respectively.
Because the ablation threshold fluence for $N$ shots, ${F_{{\rm th}}}(N)$, can be represented using the single-shot ablation threshold fluence ${F_{{\rm th}}}({1})$ as:
representing the incubation effect [28], the incubation exponent, $s$, and ${F_{{\rm th}}}({1})$ were determined to be $s = {0.34} \pm {0.09}$ and ${F_{{\rm th}}}({1}) = {14.0} \pm {8.5}\;{{\rm mJ/cm}^2}$, corresponding to the peak intensity of $({3.5} \pm {0.9}) \times {{10}^{11}}\;{\rm W}/{{\rm cm}^2}$ when the pulse duration is 40 fs, by the least-squares analysis using the ablation threshold fluences obtained for the four different values of $N$ ($N = {200}$, 500, 1000, and 2000) at a repetition rate of 1 kHz. By fitting the data plotted in Figs. 3(a) and 3(b) in the evaporation regime to Eq. (1), we determined the evaporation threshold fluence and the effective penetration depth for the PMMA sample to be ${F_{{\rm th}}}({200}) = {0.03} \pm {0.01}\;{{\rm mJ/cm}^2}$ and $({1/}{\alpha _{{\rm eff}}}) = {0.048} \pm {0.017}\;{\rm nm}$, respectively, and those for the metal nano-particle resist to be ${F_{{\rm th}}}({200}) = {0.01} \pm {0.004}\;{{\rm mJ/cm}^2}$ and $({1/}{\alpha _{{\rm eff}}}) = {0.007} \pm {0.002}\;{\rm nm}$, respectively.For the PMMA sample, the effective penetration depth in the ablation regime, $({1/}{\alpha _{{\rm eff}}}) = {0.27} \pm {0.12}\;{\rm nm}$, is much larger than that in the evaporation regime, $({1/}{\alpha _{{\rm eff}}}) = {0.048} \pm {0.017}\;{\rm nm}$, indicating that the penetration depth increases during the incubation process. The effective penetration depths of the metal nano-particle resist in the ablation and evaporation regimes are 6 times and 6.9 times smaller than those of the PMMA, respectively, reflecting the fact that the absorption coefficient of the metal nano-particle resist composed of ${{\rm ZrO}_2}$, ${0.12}\;{{\rm nm}^{- 1}}$ [29], is 5 times as large as that of the PMMA, ${0.024}\;{{\rm nm}^{- 1}}$ [29], in the wavelength range of the HHs.
In order to investigate the chemical modification in the hole area of the PMMA sample associated with the ablation induced by the irradiation of the XUV pulses, we conducted Raman microscopy measurements using a Raman microscope (JASCO NRS-5500) in the wave number range between ${1350}\;{{\rm cm}^{- 1}}$ and ${1850}\;{{\rm cm}^{- 1}}$. For the sample preparation, we irradiated the PMMA sample with the 200 XUV pulses and created a hole by the ablation process at the three different fluences of ${0.13}\;{{\rm mJ/cm}^2}$, ${0.65}\;{{\rm mJ/cm}^2}$, and ${2.48}\;{{\rm mJ/cm}^2}$. In the Raman microscopic measurements, we irradiate the hole area with pump laser pulses ($\lambda = {532}\;{\rm nm}$), whose spot size is about 1 µm in diameter, which is comparable with the hole diameter at $z = {14.6}\;{\unicode{x00B5}{\rm m}}$.
In Fig. 4, the recorded Raman microscope spectra are shown for the area of the sample that was not irradiated with the XUV pulses and the hole areas processed by the XUV pulses having the three different fluences at $z = {14.6}\;{\unicode{x00B5}{\rm m}}$. It can be seen in Fig. 4 that the intensity of a peak located at ${1730}\;{{\rm cm}^{- 1}}$ decreases gradually as the XUV fluence increases, and a peak start appearing at ${1640}\;{{\rm cm}^{- 1}}$ when the sample is irradiated with the XUV pulses and its intensity increases as the XUV fluence increases. The fluence dependence of the Raman microscope spectra suggests that, by the irradiation of the XUV pulses, the C–C bonds in the polymer chains in the PMMA sample are partially cut, the ${\rm C} {=} {\rm C}$ double bonds are formed, the ${\rm C} {=} {\rm O}$ chemical bonds are partially broken, and/or the moieties containing the ${\rm C} {=} {\rm O}$ bond are dissociated [30–32].
Fig. 4. Raman microscope spectra of the PMMA sample. The black line represents the Raman microscope spectrum of the area of the PMMA sample that was not irradiated with the XUV pulses. The blue, green, and red lines represent the spectra of the holes created by the XUV irradiation when the fluences are ${2.48}\;{{\rm mJ/cm}^2}$, ${0.65}\;{{\rm mJ/cm}^2}$, and ${0.13}\;{{\rm mJ/cm}^2}$. The peaks at ${1730}\;{{\rm cm}^{- 1}}$, ${1640}\;{{\rm cm}^{- 1}}$, and ${1450}\;{{\rm cm}^{- 1}}$ are assigned to the vibrational modes of the ${\rm C} {=} {\rm O}$ stretching, ${\rm C} {=} {\rm C}$ stretching, and ${{\rm CH}_2}$ bending, respectively. The peak appearing at ${1556}\;{{\rm cm}^{- 1}}$ marked with an asterisk is assigned to the vibrational frequency of ${{\rm O}_2}$ in the air.
In summary, we focused ultrashort XUV pulses (27.2–34.3 nm) generated by HHs generation using near-IR femtosecond laser pulses using an ellipsoidal mirror fabricated and designed to focus optics in the XUV region on a surface of two kinds of solid thin films, and we demonstrated that the processing of the PMMA sample and the metal nano-particle resist in a sub-micrometer scale is possible in the ablation regime and that ultrafine processing and chemical modifications of solid materials in the sub-micrometer scale have become possible by the sub-micrometer focusing of the femtosecond-laser-based XUV pulses.
Funding
MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) (JPMXS0118067246, JPMXS0118068681, JPMXS118070187); New Energy and Industrial Technology Development Organization; Japan Society for the Promotion of Science (15H05696); JSPS Strategic Fund for Strengthening Leading-Edge Research and Development; Center of Innovation Program.
Acknowledgment
The authors thank Dr. Toshiro Itani and Dr. Julius Joseph Santillan (EIDEC Inc.) for their support in preparation of the metal resist sample.
Disclosures
The authors declare no conflicts of interest.
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