A novel technique is introduced that dramatically increases the quality and spatial resolution of directly ablated periodic nanostructures on materials. The presented method utilizes a PMMA confinement layer spin coated on the surface of the ablated material reducing the violence and speed of expansion of the molten material. As a result, droplet formation deteriorating the achievable resolution can be completely avoided. Moreover, motion control of the molten material leads to structural details with dimensions well below the irradiation wavelength.
© 2013 OSA
Nowadays, micro-electronics, micro-mechanical, or micro-optical components are embedded in virtually all high-tech products. Therefore, material processing at the submicron level has become a key issue in the fabrication of advanced technology devices.
In mass production, for the fabrication of high spatial resolution (< 100 nm) structures the standard way is to use lithographic techniques. However, in particular for rapid prototyping, there is a growing need for alternative techniques offering a higher level of flexibility. Direct laser processing based on surface ablation, being one such alternative, provides versatile fabrication capabilities for a lot of materials. Unfortunately, the spatial resolution of this technology is limited by diffraction to the level of the applied laser wavelength (typically a few hundred nanometers). Furthermore the process of material removal itself may deteriorate the quality of high resolution structures. In recent years, the self-organized nanogratings (ripples) on various materials including metals, semiconductors and dielectric materials induced by laser irradiation have attracted much attention [1–5]. According to these reports, structures with feature sizes smaller than 100 nm can be easily achieved. However, the patterns obtained in all these cases exhibit statistical irregularities of their periodicity, thus limiting the eligibility of this approach for several applications.
In this paper we introduce a new technique and demonstrate its ability to create high quality patterns via laser direct writing of regular, strictly periodic structures. As a result, we are able to fabricate structural details with dimensions being only a fraction of the applied laser wavelength. Moreover, the technique helps avoiding debris formation on the processed surfaces leading to clean, high definition structures.
When applying laser processing for the fabrication of submicron structures some key considerations have to be made. It has long been known that in case of materials with high thermal diffusivity like e.g. metals or semiconductors (especially silicon) ultrashort pulses should be used to maintain sufficient spatial concentration of the deposited energy . As a rule of thumb, the maximum allowable pulse duration (τ) can be calculated as (τD)1/2, with D being the thermal diffusivity of the material. For some materials this means that pulse durations below 1 ps should be applied if structural details on the level of 100 nm are to be fabricated.
On the other hand, at such pulse durations (below 1 ps) and at fluences well exceeding the ablation threshold, the process of material removal is rather complex, including photomechanical spallation, explosive disintegration of the overheated surface, and droplet ejection. These highly dynamic processes were investigated by a number of groups using molecular dynamic (MD) simulations [7–10]. A simplified explanation for the process of droplet ejection is displayed in Fig. 1. In practice, during ablation of metals with femtosecondpulses, we indeed observe the ejection and resolidification of droplets with typical sizes ranging from 50 to 100 nm (Fig. 2) , similar to the predictions of theoretical model calculations. Evidently, this behavior fully prohibits the controlled fabrication of high definition submicron structures.
2. The concept of confinement
The key idea to avoid the formation of melt droplets is to prevent the irregular expansion and surface breakup of the molten phase. This can be done with the help of a thin transparent “confinement”-layer deposited on the surface of the target material.
The concept of laser-produced plasma in confined geometry, first introduced by Fabbro et al , has long been known to enhance momentum transfer by plasma expansion during laser ablation. With this technique, a laser irradiates a target at an intensity exceeding the ablation threshold, and the produced plasma is confined by a transparent layer covering the target. This configuration has been proven to offer great advantages for various applications where enhanced shock pressures are of importance.
In this paper we demonstrate that a transparent confinement layer can be used to achieve periodic surface patterns with structural details of superior resolution. The irradiating light pulse traverses the transparent layer with negligible attenuation and hits the strongly absorbing target material causing rapid melting of its surface. Due to material expansion, the driving force of the bulk material tends to expel the molten material causing ablation. However, the adjoining transparent layer on top exerts recoil pressure and prohibits free expansion of the molten material, thus acting as a confinement layer (Fig. 3). This confinement effect results in a cohesive molten zone with a strongly reduced expansion velocity. In this way droplet ejection can completely be avoided, leading to a smooth resolidified surface.
3. Results and discussions
As already mentioned, a prerequisite for submicron precision laser processing of materials with high thermal diffusivity is a pulse duration in the picosecond or even subpicosecond range. A further increase of the spatial resolution can be achieved by applying short wavelength radiation. Thus, in terms of structural details, the highest possible resolution is expected by applying UV femtosecond pulses. Therefore, in the experiments a UV short pulse laser system was applied delivering multi-millijoule pulses at 248 nm with a typical pulse duration of 500 fs . The optical scheme for sample irradiation is displayed in Fig. 4.
After passing a variable attenuator for energy adjustment, the laser beam is directed onto a transmission grating (chrome on quartz) carrying a two dimensional crossed grating structure with a period of 20 µm. A quadratic aperture of 1 mm is placed directly behind the grating serving as a mask. A beam selector is used to block all diffracted beams emerging from the grating except the four first order beams (representing a boxcar configuration). A Schwarzschild-type reflective objective (Ealing x 15, Na 0.28) was applied to create an image of the aperture mask on the sample surface with a demagnification of 15 – 20. In this scheme mask projection is combined with multiple beam interference, allowing parallel processing of submicron structures over an extended sample area [14, 15]. The use of reflecting optics prevents any pulse front distortion and the imaging setup ensures that the sample is placed outside the focal region, therefore no spatio-temporal issues arise even if ultrashort pulses are applied.
As a next step, we have to identify a suitable confinement layer. As already mentioned, the effect of confinement is well known in the literature. In numerous studies a liquid environment (typically water) was applied. In our case, however, where UV ultrashort pulses are used in a geometry, in which coherent beams are superimposed on the sample surface to create a diffraction pattern, the optical quality of the confinement layer plays a decisive role. In case of a liquid layer, it is rather difficult to maintain a sufficiently thin layer of good optical quality. As an alternative, transparent polymers can be spin-coated, exhibiting a virtually perfect optical quality with a very precise thickness control. For our wavelength of 248 nm PMMA was selected based on its excellent transmission down to the UV. The thickness of the layer was chosen to be 400 nm, representing a good compromise between high enough transmission (> 90%) and sufficient inertia to provide the necessary recoil pressure for the confinement. An example for UV-fs laser ablation of sub-µm periodic structures on nickel is shown in Fig. 5. Without the confinement layer, ejected and resolidified droplets dominate the structures and dramatically reduce the achievable spatial resolution.
Applying a confinement layer, the resolidified droplets are strongly suppressed resulting in a dramatic improvement of the pattern formation.
Another advantageous effect of the presented technique is the prevention of debris formation on the ablated surface as shown for silicon in Fig. 6. This can be explained by considering that small-sized hot fragments ejected from the ablated surface at very high velocities penetrate the confinement layer and become trapped there. Moreover, for silicon the melt droplets are completely absent and the single features are sharp and clean. The even higher surface quality as compared to nickel can be explained by the observation that the formation of melt droplets on semiconductors is much less pronounced than in case of metals.
The forces acting on the confinement layer finally cause full detachment of the layer from the surface of the substrate material as shown in Fig. 7. Therefore this technique is only applicable for single pulse irradiation. In order to extend the presented technique to multiple pulse ablation experiments, a flowing liquid film as confinement layer should be applied. In that case further development is needed to ensure appropriate thickness and sufficient surface quality of the liquid layer.
The key advantage of the confinement technique is, however, the capability of forming structural details that are much smaller than the applied irradiation wavelength. This particular feature of the novel technique can be understood by taking into account the dynamic material flow beneath the confinement layer. Considering a simple irradiation pattern composed of a two dimensional array of intensity maxima, the resulting topology on silicon is as displayed in Fig. 6. Upon increasing the fluence, the ablated structures get bigger and the intact surface between them gets smaller as shown in Fig. 8(a). By further increasing the fluence one gets to a regime where the free space between the adjacent ablated spots vanishes. In this case the molten material driven out of the center of each individual ablated spot is getting pushed to the edge of the unit cell which is repeated along a two dimensional pattern. Since in all neighboring unit cells the same scenario is taking place, the topology displayed in Fig. 8(b) is obtained.
This effect gives rise to the establishment of patterns with structural details well below 100 nm as seen in the figure. Evidently, applying other laser field distributions and controlling the laser fluence, a variety of different high resolution patterns can be obtained. A further example is displayed in Fig. 9. In this case a liner grating structure was projected onto the sample surface. Similarly to the situation presented in Fig. 8(b) but now just in one dimension, the fluence can be increased to a level at which the molten material is driven to the boundary of the grooves. Getting very close to each other, the resolidified material at each side of the irradiated zones forms pairs of long stripes.
In conclusion, we introduced a novel technique that dramatically increases the quality and spatial resolution of directly ablated periodic nanostructures on materials that exhibit thermal ablation behavior. The presented method utilizes a PMMA confinement layer spin coated on the surface of the ablated material reducing the violence and speed of expansion of the molten material. In this way droplet formation that strongly decreases the achievable resolution of the ablated structures can be completely avoided. Moreover, the controlled and confined expansion of the molten material leads to new structures with superior resolution well below the irradiation wavelength.
The authors are indebted to Jürgen Ihlemann for stimulating discussions. This research was partly supported by the DFG under the contract IH 17/18-1.
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