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Use of high numerical aperture focusing with negative longitudinal spherical aberration is shown to enable deep (>10 µm), high aspect ratio, nano-scale-width holes to be machined into the surface of a fused-silica (SiO2) substrate with single pulses from a 200 fs, 4 µJ Ti-Sapphire laser source. The depths of the nano-holes are characterized by use of a non-destructive acetate replication technique and are confirmed by imaging of sectioned samples with a dual focused ion beam/scanning electron microscope.
©2008 Optical Society of America
1. Introduction
Femtosecond laser materials processing is a versatile tool for precise machining of micro- and nano-scale features, as there is little or no damage from generation of stress waves, thermal conduction, or melting [1–3]. Direct laser writing of patterns enables rapid prototyping without photo-masks, molds, and post-processing. Some important applications of ultrafast lasers include fabrication of photonic and fluidic devices: waveguides, couplers, optoelectronic systems, information storage, and fluidic channels [4]. Laser nano-structuring of transparent polymers and glasses are of interest for lab-on-a-chip devices for biotechnology applications [5–7] while fused silica (SiO2) is particularly relevant for fluidic devices for single-molecule fluorescence detection due to its low autofluorescence background [8].
Fused silica shows low linear absorption at Ti:Sapphire laser wavelengths (700–950 nm), but non-linear absorption becomes significant when ultrashort laser pulses are tightly focused by a high numerical aperture lens onto the surface or inside the bulk [9]. Then, the intensity in the focal volume becomes sufficient to initiate nonlinear absorption processes, which lead to optical breakdown, micro-plasma formation, and permanent changes in the refractive index of the material [10–12] or removal of material [13–15]. These processes have a highly-nonlinear dependence on the instantaneous irradiance and hence, if the laser pulse energy is adjusted so that only a small part of the focused beam has irradiance above the threshold for material modification or ablation, sub-diffraction limited features can be machined [1,16–18].
The mechanisms and dynamics of the damage produced by femtosecond pulses have been extensively studied and it is generally the case that optically induced breakdown proceeds by avalanche ionization [19,20]. For pulses longer than about a picosecond, the damage threshold exhibits a square-root dependence on the pulse duration and stochastic behaviour, while for shorter pulses there is a deviation from the square-root dependence and the behaviour becomes highly deterministic [21]. Until recently, it was commonly believed that for shorter pulses multiphoton ionization plays a dominant role in the generation of seed electrons for the avalanche ionization and is responsible for the deterministic nature of the damage threshold. However, detailed studies close to the breakdown threshold have provided strong evidence that multiphoton ionization does not play a significant role, but that the dominant mechanism is a combination of Zener ionization (tunnelling) and Zener-seeded saturation avalanche ionization [22,23]. The deterministic nature and reproducibility of the optical damage is thereby explained by the high and uniform valence electron density in the target material.
Femtosecond laser creation, relocation, and merging of optical voids inside bulk silica glass have been demonstrated [24,25]. Recently, high-aspect voids of 200 nm diameters and about 20 µm in length have been created 20–70 µm beneath the surface, and have been attributed to self-focusing of the beam as it propagates from the surface [26]. The voids are shorter if the laser pulse is focused closer to the surface, and they become ~3 µm in length for focusing at a 20 µm depth. These studies have also shown that multiple-pulse irradiation breaks the line-shaped void into multiple voids [26]. The influence of the laser repetition rate on heat accumulation and refractive index modification has also been investigated [14,27]. A compaction of the material surrounding ultrashort-pulse driven micro-explosions resulting in ~200 nm diameter cavities internal to the substrate has been demonstrated and attributed to strong self-focusing [28]. Creation of long filaments due to self focusing of femtosecond pulses within transparent media has been extensively studied [29,30]. Filamentation within the ablated material plasma in multi-pulse femtosecond hole drilling has been proposed as a mechanism for creation of high aspect ratio features [31].
In addition to self focusing, spherical aberration (SA) in the focusing optics also modifies the irradiance profile around focus and elongates the focal region in the direction of propagation [32,33]. Most commercially available high-NA microscope objectives are designed to minimize SA for imaging through a borosilicate coverglass of refractive index n=1.522 at 588 nm wavelength and thickness of 0.17 mm. Hence if a femtosecond laser pulse is focused at a different depth or into a material with a different refractive index, such as fused silica with n=1.453 at 800 nm, the focusing will exhibit SA, which causes an increased breakdown threshold and degraded spatial resolution [34]. The femtosecond laser writing of deep sub-surface waveguides has been found to be significantly affected by SA and means for corrections to produce waveguides with more circular cross-sections have been studied [35–40]. Interface-induced SA and initial SA due to incorrect beam collimation at the objective have been found to be important factors that combine with non-linear self focusing of a femtosecond pulse for creation of filamentary plasmas deep within fused silica [41]. Longitudinal chromatic aberration in focusing a dual-color 800/400 nm femtosecond beam has been shown to improve the multi-pulse drilling of deep sub-millimeter-diameter holes at the surface of a transparent material [42].
This paper presents single-pulse femtosecond laser drilling of high aspect ratio sub-micron diameter holes that start at the surface of the SiO2 substrate, as is crucial for creation of nanofluidic devices. Section 2 contains a description of the experimental system, in which femtosecond pulses are focused at the surface with a high numerical aperture objective with negative spherical longitudinal aberration. The experimental results are presented in Sec. 3. Holes exceeding 11 µm in depth and diameters at the surface of 200–500 nm have been created. The dependence of the machined structures on the laser pulse energy is reported. An acetate sample replication technique is used to estimate the depths of the nano-holes. The validity of the replication technique for characterizing such high aspect ratio features is confirmed by focused ion beam (FIB) sectioning of the nano-holes followed by visualization with a scanning electron microscope. In Sec. 4, possible mechanisms for femtosecond pulse creation of deep nano-holes beginning at the surface, due to strong focusing with spherical aberration and self-focusing of the off-axial energy are briefly discussed. Section 5 summarizes this research aimed towards femtosecond laser fabrication of nanoscale devices.
2. Experimental setup
The femtosecond machining source used in this work is a Ti-Sapphire amplifier (Coherent, Inc., RegA 9000), operated at a center wavelength of λ=800 nm, with repetition rate of up to 250 kHz, pulse width of τ≈200 fs (FWHM), and average power of 1 W. The beam power at the sample is controlled by a variable neutral density filter and the Pockels cells within the amplifier are externally triggered to enable the selection of a single laser pulse. The laser pulse is focused onto the sample using a dry microscope objective (Nikon CF Plan Achromat 79173) with a numerical aperture of NA=0.85 and working distance of 0.41–0.45 mm. This objective is an older type designed for a conjugate of 150 mm. The beam is expanded using a pair of lenses so that it fills the entrance pupil of the objective and is also focused 150 mm prior to the objective mount. The objective has a correction collar to adjust for spherical aberration equivalent to a standard coverglass of thickness of 0.11–0.22 mm and refractive index 1.52. However, for creation of high aspect ratio holes at the surface, the laser beam is directly focused without a coverglass onto the first surface of the substrate of fused silica, which has refractive index 1.453 at 800 nm, with the collar set to 0.17 mm. Accordingly, the beam focusing for these experiments has negative (under-corrected) longitudinal spherical aberration. A model of the focusing of the Gaussian laser beam, which uses optical design software (Zemax Development Corporation) to create with idealized paraxial surfaces a system for which perfect focusing would be achieved through a coverglass, which is then replaced by an adjustable thickness of air, indicates that the profile at the paraxial focus is expected to have a micron-sized central peak, which arises from a paraxial ray bundle, and a considerable fraction of the pulse energy in the wings beyond this, which arise from the marginal rays. An experimental measure of the attenuated beam made by translation of a 1-micron diameter pinhole and photodetector across the focal region confirms that the central peak is approximately 1 µm in diameter.
The sample is positioned within the focus of the beam with nanometer precision by use of a piezoelectric stage with 200 µm range of motion in x, y, and z directions (Mad City Labs, Inc.), and this in turn is positioned on a large range of motion x, y microstage (McBain, Inc. precision stages and Compumotor, Inc. stepper motors with National Instruments motor drivers). The objective lens is mounted on a z-axis translation stage with a LabView-controlled motorized micro-stepper (Oriel, Inc.). To focus onto the first surface of the sample, the laser is greatly attenuated and the objective is scanned in the z-direction, while reflected light is imaged onto a silicon photodiode detector (Perkin-Elmer HUV 2000). A collinear Helium Neon laser is also available for raster scanning the sample after processing to obtain the relative coordinates of created features within the coordinates of the microstage. Samples consist of 200 µm thick fused silica wafers. These are positioned on an aluminum sample holder attached to the nano-positioning stage, which is moved in a pre-programmed x, y pattern under LabView control. All samples are cleaned in an ultrasonic bath with acetone, methanol, and deionized water, and dried with nitrogen before the experiments, which are carried out at room temperature and atmospheric pressure in a Class-1000 clean room.
3. Experimental results
3.1 Nano-hole size dependence on laser energy
Figure 1 shows several scanning electron microscope (SEM) images of features, each machined by a single femtosecond pulse at different pulse energies. All samples were sputter-coated with a thin 20–40 nm layer of gold to provide conductivity for SEM examination. The dark circles in the middle of the white halos are the nano-holes, each produced by a single pulse, while the white halos are due to redeposited material removed during machining. The difference in contrast is attributed to different electrical conductivities of the redeposited material and the sample surface. In Fig. 1(d), parts of the white halo have peeled off during post machining ultrasonic sample cleaning, prior to gold coating, revealing the underlying gray sample surface. Similar redeposited material is reported in Ref. [23].
Fig. 1. Nano-holes machined by single laser pulses at different energies: (a) E=1.20 µJ; (b) E=1.6 µJ; (c) E=2.40 µJ; (d) E=1.20 µJ.
Figure 2 shows the dependence of the nano-hole diameter at the sample surface on laser pulse energy. The smallest surface features observed with the SEM were machined at a pulse energy of E≈0.1 µJ. If the beam were diffraction-limited, the calculated threshold energy flux would have been U th≈25 J cm-2, which is ~10 times higher than the energy flux used in a previous report describing machining of fused silica with similar pulsewidths [43]. This difference emphasises that a considerable portion of the pulse energy is actually contained within wings that surround the central focal spot, due to the focusing with spherical aberration.
While it is straight forward to measure features at the surface, it is not so easy to characterize the depth of high aspect ratio features. The conventional technique for feature depth estimation is atomic force microscope (AFM) measurements. However, this method would have difficulty in obtaining the signal from the bottom of a nanometer-size, high aspect ratio feature [28]. Therefore, a replication method is used to estimate the depth of the nano-holes. This is a well established technique for SEM and transmission electron microscopy (TEM) measurements of different materials [44–46]. Sample replication provides a fast, non-destructive, and inexpensive technique to obtain information about sub-micron structures on and below the sample surface. Commercially-available cellulose-based acetate films (35 µm-thick cellulose acetate film from Electron Microscope Sciences, Inc.) were used. In order to soften the acetate film, a drop of acetone is applied to the sample surface. The film is then applied to the sample surface to replicate the laser-machined features. As the acetate films are easily removable, the original sample is preserved. The extracted replicas are then coated with a layer of gold for examination with the SEM, as described above.
Figure 3 shows the replicas of an array of nano-holes, each machined by a single laser pulse at different energies. The results indicate that the nano-holes have the desired high aspect ratio and a smooth structure. Some of the replicas are bent, either during or after the extraction, or during gold coating.
Fig. 3. SEM images of replicated arrays of nano-holes (45 degree tilt): (a) Nano-holes machined with laser pulse energy E=1.60 µJ; (b) E=2 µJ.
Figure 4(a) shows the dependence of the nano-hole depth on the laser pulse energy as estimated by acetate replication. The SEM stage tilt (45 degrees in Figure 3) has been taken into account. At low laser energies close to the ablation threshold the nano-holes are quite shallow, in accord with prior results (40–400 nm [17,23,28]). The smallest nano-hole structures detected by the SEM were machined at the laser pulse energy E ~0.8 µJ. The depth rapidly increases when the laser energy exceeds 1.5 µJ. Overall; the increase in nano-hole depth with laser pulse energy (~12 µm) is more than an order of magnitude greater than the increase of nano-hole diameter (~0.3 µm). The dependence of the nano-hole aspect ratio (length/diameter) on laser pulse energy is shown in Fig. 4(b). Note that the length of the nano-holes significantly exceeds the skin depth of absorption in the laser-induced plasma (~30 nm [23]) and also the Rayleigh range that would be obtained from a diffraction-focused Gaussian (~1.4 µm). Also the surface diameter of the nano-holes is smaller than the diffraction-limited spot size (≈ 1.22λ/NA=1.1 µm) even though the beam was focused with spherical aberration.
Fig. 4. Dependence of (a) hole depth, and (b) aspect ratio, as obtained by acetate replication, on the pulse energy.
3.2 Nano-hole depth sectioning by DualBeam™ SEM/FIB
While the replication technique is well proven for nano-particle extraction and surface feature replication [44–46], there was concern that the high aspect ratio features may resist accurate replication. The replica of a nano-hole is a nano-wire with high surface area to volume ratio and any sticking within the hole might cause stretching during extraction. However, in each experimental condition, over 600 nano-hole replicas were extracted, and the replicas are remarkably uniform in length and diameter whereas sticking should produce a more stochastic result. Albeit, in order to definitively characterize their depths and cross sections, selected nano-holes were measured by direct SEM imaging following focused ion beam (FIB) sectioning. A DualBeam™ SEM/FIB tool (FEI Nova 600 NanoLab™) was used to section the nano-holes in fused silica. This tool has been widely applied in precision cross-sectioning, TEM sample preparation, and automated 3D process control [47]. It integrates both a focused ion beam (FIB) and scanning electron microscope (SEM) in a single system, so that the results of the ion beam milling can be immediately characterized by SEM image analysis. As shown in Fig. 5, the ion beam bombards the sample at an angle of 52° to the surface normal to produce a trench with desired geometry. The FIB milling mode “cross sectioning” was used to produce an ion beam-polished flat surface, denoted as BC in Fig. 5(a), which sectioned several nano-holes in its path. After FIB machining, the SEM was used to observe a projected image on AC of the sectioned nano-holes on the plane BC. The depth AB is calculated as AB=AC/tan 52°=0.78 AC, where AC is measured from the SEM image.
SEM images of an ion beam milled trench are shown in Fig. 6. The distance (AC) of a nano-hole from the entrance point of the ion beam, and the section depth (AB) for the four nano-holes are listed in Table 1. The section depth for nano-hole #4 is 11.7 µm, which definitively proves that the nano-hole is deeper than this, as it is still visible following sectioning to this depth. The FIB sectioning confirms that the extreme depth of the nano-holes is not overestimated by the replication technique. For this sample, created with pulse energy of 2.4 µJ, the average hole-depth estimated by the replication technique was 9.5 µm. Hence, the replication technique most probably underestimates the depths. The difference might be due to the difficulty of the polymer to reach the bottom of the nano-hole and/or in distortion of the acetate nano-wires during gold coating, as is apparent in Fig. 3.
Fig. 5. (a) Schematic side-view diagram of the DualBeam™ SEM/FIB tool. The angle between the electron beam and the ion beam columns is 52°. (b) The scope image inside the chamber of the tool.
Fig. 6. (a) SEM image of the sectioned nano-holes #1–4 in the trench at zero degree tilt. Note that AC is a projected length on the sample surface. All 4 holes are still visible within the trench. (b) View of the trench in (a) after the sample was rotated 90° clockwise then tilted 25°, with nano-holes #1 and #2 in view. The triangle ABC corresponds to that in Fig. 6(a).
Table 1. Minimum depths of nano-holes in Fig. 6(a)
In Fig. 6(a), the diameters of the holes appear to be smaller at greater depths. The general shape of the top of each hole is more clearly visible in Fig. 7, which was produced by positioning the ion bean so as to section close to a line of nano-holes. A region with four nano-holes was milled, of which nano-hole #2 was sectioned at the top to expose a long, thin, conical-shaped entrance profile.
Fig. 7. SEM image at 52 degree tilt of FIB cross-sectioned nano-hole entrance. The sample surface was gold coated for milling.
4. Discussion
The long, thin, conical entrance profile seen in Fig. 7 suggests there is self-focusing of the laser pulse as it propagates from the surface to form a deep hole with a sub-diffraction sized diameter. The creation of similar long features internal to the material has been observed and attributed to self-focusing [26,28] due to the Kerr nonlinearity of fused silica, or due to interface-induced spherical aberration (SA) combined with self-focusing [41]. However, in these previous works the features become lengthened only when created >20 µm from the surface, in part because interface-induced SA (i.e., SA resulting from uncompensated surface refraction) becomes negligible when focusing near the surface. In contrast, in Fig. 7 the femtosecond pulse is focused at the surface with an initial SA due to the objective. Accordingly, due to SA, the irradiance profile at the surface has wings surrounding a central peak and the focal volume is extended in length compared to the Rayleigh range of a diffraction focused beam [32,33]. An estimate of the profile obtained by using optical design software (Zemax Development Corporation) to model the focusing of a Gaussian beam with an ideal 0.85 NA lens, with spherical aberration equivalent to that due to removing a 0.17 mm coverglass, indicates that the cross-section of the Huygens point spread function contains a micron-diameter peak on axis with wings extending several microns from the axis and that the central peak extends about ~10 µm in depth.
With this irradiance profile, the 200 femtosecond pulse leads to ionization of the material at the center of the axial peak to form a plasma, but motion of this plasma during the pulse is negligible [20], and ejection of the hot plasma out from the surface of the substrate or into the surrounding material would occur many microseconds later. On axis, the increase in free-electron density produced by Zener-seeded avalanche ionization during the laser pulse augments the plasma frequency so that the plasma becomes strongly absorbing and then reflecting when the plasma frequency exceeds the laser frequency [20], so that only the initial part of the laser pulse is expected to be transmitted beyond the surface. However, due to spherical aberration, and with contribution from the Kerr non-linearity of the fused silica, the energy in the wings of the focused beam come into focus on axis beneath the surface, thereby bypassing the opaque on-axis generated plasma. The Kerr nonlinearity of fused silica has a response time of 10-16 s due to laser distortion of electronic wavefunctions [48] and n 2=2.5×10-16 cm2 W-1 [29]. Hence the response is instantaneous on the time scale of the 200 femtosecond laser pulse. Also, the estimated peak irradiance level in the wings is ~1-5×1013 W cm-2, which yields a Kerr self focusing angle of about 3–6 degrees [49], so that self-focusing is likely to contribute to directing the energy in the wings of the focal region towards the axis.
5. Summary
Femtosecond lasers provide an enabling means for direct writing of nanometer-scale features. There is interest in fluidic and photonic component fabrication strategies for waveguides, micro- and nano-channels and nano-holes, especially in materials suitable for lab-on-a-chip devices. Fused silica is particularly of interest because it has low autofluorescence at visible wavelengths, making it attractive for ultra-sensitive fluorescence studies. In this research, conditions for machining vertical nano-holes starting at the surface in transparent materials such as fused silica, of depths exceeding 11 µm and diameters of 200–500 nm, using a single 200 femtosecond laser pulse have been experimentally identified. This is of significant utility for creating novel devices for single-molecule studies [8,50]. Focusing the femtosecond laser pulse with a high numerical aperture objective with spherical aberration is a key element for creating the high aspect ratio features, although self-focusing due to Kerr non-linearity is also expected to play a role. A simple non-destructive replication technique for estimating the dimensions of the nano-holes has been demonstrated. The validity of the replication technique has been established by direct imaging of nano-holes with a scanning electron microscope following sectioning with a focused ion beam.
Acknowledgments
This work is supported by the Center for Laser Applications at University of Tennessee Space Institute and DARPA grant W911NF-07-1-0046. We thank Alexander Terekhov for essential technical support in conducting this research. The DualBeam™ analysis was conducted at the Center for Nanophase Materials Sciences and the SHaRE User Facility, which are sponsored by the Division of Scientific User Facilities, Office of Basic Energy Sciences, U.S. Department of Energy.
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