Abstract
Current developments and challenges in laser patterning
Patterning in the micro and nano-scale is essential to many product innovations. They lead to mass-productive and cost-effective processes related to various research fields such as nanotechnology, biotechnology, information technology, and micro/nanofluidic devices and systems.1 – 3 The ability to realize device miniaturization can be mainly attributed to the advances in precision engineering techniques, which are able to fabricate ever-smaller functional micro-/nanostructures in a cost-effective way.4
One of the most successful methods in microfabrication is Optical lithography. Optical lithography has been widely used for patterning sub-micrometer features, generally up to 100 nm, due to the physical diffraction limit. Currently, the use of deep UV lasers has allowed the improvement of optical resolution below 100 nm.5 For example, deep-UV wavelength excimer laser light sources have driven lithography resolution to about 60 nm half-pitch for dry exposures and about 40 nm for 193 nm immersion exposures. The traditional laser light source drivers at each technology node have been higher power and smaller bandwidth. High power enables increased productivity and lower cost per layer as scanner stage speed increases. Small bandwidth minimizes contrast degradation due to chromatic aberration in the projection lens, which becomes more challenging at higher numerical aperture.6
As a logical extension of optical lithography, extreme ultraviolet radiation (EUV) has been considered for next generation of lithography devices.7 However, the employed EUV radiation (e.g. 13·5 nm wavelengths) is strongly absorbed by solid-state materials and even by air. Beside long-term stability source, the most difficult technological challenge for EUV is a special reflective mask, with tens of layers of reflective coatings each.
Although these processes have many advantages and usefulness, they require inevitably complex procedures and high cost photomask for precise patterning.
Recently, increasing trends toward direct nano- and submicrometer patterning processes which refer to the fabrication of patterns without photomasks have been reported. In particular, laser based technologies provide unique advantages of being a noncontact process in a flexible setup that can operate in air, vacuum or liquid environment, making it very attractive as a processing and manufacturing tool in extended applications. Laser radiation can be easily focused down to the micrometer scale and has found its place as established tool in modern industries, allowing different processes such as marking, drilling, annealing and surface modification as well.4
In order to achieve both high fabrication speeds and high patterning resolution, appropriated laser sources must to be utilized. Nowadays, there is a wide palette of laser systems which permit processing of materials with femtosecond (fs), picosecond (ps) or nanosecond (ns) pulses as well as different wavelength (e.g. UV, VIS or IR). One of the key parameters to evaluate the quality of the laser process is given by the heat-affected zone, which basically depends on thermal properties of the materials as well as the pulse duration. In order to go down to the nanoscale, ultra fast laser systems (fs and ps) have been utilized recently.8 In particular, femtosecond laser machining has been proven to be a highly efficient tool for microstructuring due to the inherent characteristics of the process, such as distinct and sharp laser energy density threshold for ablation significantly lower than that of nanosecond or longer pulses, and minimization of shock-affected areas, and multiphoton absorption processes.8
In the past few years, some works have been carried out in nanoscale fabrication technology using two-photon polymerization (TPP) induced by a femtosecond laser. It is well known that TPP has many advantages as a technique for direct fabrication of complex three-dimensional (3D) structures on a scale of several microns, which might be difficult to obtain using the conventional technologies. When fs-laser radiation is closely focused into the volume of liquid-state monomers, the laser pulses can initiate the chemical process for polymerization with the resolution of approximately 100 nm based on non-linear interaction processes. This feature size is less than diffraction limit of a laser beam.9 By employing this technology, various 3D microstructures with applications as optical memories, photonic crystals, micro-rotors, micro-oscillators have been reported.
Another way to overcome the diffraction limit using laser radiation is related to the generation of evanescent waves allowing enhancement of the electric field. This can be accomplished by combining scanning probe microscopy (SPM) devices with ultrashort laser systems. SPM allows the characterization of surfaces morphologies with a resolution below 1 nm using a physical probe (tip) that scans the specimen. For the nanopatterning setup, the tip is irradiated with the laser source (e.g. fs-laser radiation) and acts as an antenna resulting in a strong electric field enhancement at the tip (∼50–200 times). This enhancement causes the thin-film materials to be removed or modified. Conventional spatial resolution of features produced using this method is close to 10 nm.10 Since the evanescent waves rapidly decay in an exponential order with the substrate separation distance, a precise control of this distance must be implemented. By operating in non-contact mode, the tip lifetime can be extended. A drawback of this technology is given by the long processing times that are required due to the slow fabrication speeds (in a range of μm/s). The same phenomena has been reproduced using other approaches like irradiation of self-assembled transparent nanoparticles (e.g. silica particles) which act as lenses11 and producing structure with features around 50–100 nm.
Another maskless and noncontact nanofabrication technique that has been developed in the last years is Laser Interference Lithography (LIL). This method has been utilized for the fabrication of periodic structures by irradiating a photoresist on a substrate (generally silicon) with two or more coherent light beams to form a horizontal standing wave pattern. After irradiation of the resist (as well as proper thermal treatments to enhance mechanical properties of the polymer), the substrates are developed and etched obtaining periodic arrays of lines or dots, depending on the configuration utilized for irradiation process. Compared to conventional sequential methods, impressive fabrication speeds can be achieved. For example, for single beam direct writing, it will take several hours (or even days) to fabricate an array of lines on some square centimetres of surface area, while the same result can be achieved in less than a second with LIL. For a line-like array, the spatial period depends on the intercepting angle between the beams and the laser wavelength. The smallest period can be then reduced to half of the laser source wavelength.12
The introduction of new high power pulsed laser systems in the market has also permitted the use of interference patterns for the direct fabrication periodic arrays in several materials including metals, ceramics and polymers.13 This technology offers the ability of conventional LIL with the additional advantage (compared to LIL) that any secondary fabrication step is necessary. In consequence, new application fields will emerge in the next years.
For about 50 years, laser radiation was an invention in search of an application. Today the laser is the foundation for communication as well as elemental and fundamental tool in several manufacturing processes. Considering the last recently developed technologies, we can be sure that we are still at the beginnings of a new era, where micro and nanofabrication techniques will be significantly different from the past, and many of them will require laser radiation.
Team Manager/Gruppenleiter
Working Group Surface Functionalization/Arbeitsgruppe Oberflächenfunktionalisierung Fraunhofer Institute for Material and Beam Technology/Fraunhofer-Institut für Werkstoff- und Strahltechnik IWS Winterbergstraβe 28
01277 Dresden, Germany
Phone: +49 351 83391 3007
Fax: +49 351 83391 3300
http://www.andreslasagni.t35.com/
A. Fabián Lasagni
