Industrial applications of excimer laser include fabrication of multi-chip modules, ink jet nozzles and TFT annealing of flat panel displays. For more than a decade these applications and the deep-UV-lithography pushed the excimer laser technology to improved performance and lower cost. As a result, highly reliable laser systems have been developed, which utilize state of the art technologies like metal ceramic laser tubes, solid state switching circuits and solid state halogen generation.High repetition rate lasers are suitable for micromachining applications especially in the direct structuring mode. Depending on the processing parameter the throughput and operating cost of such a high repetition rate system will be advantageous compared to standard laser systems. In the absence of other process inherent limitations, the processing time both for 2D and 3D laser ablation are proportional to the lasers pulse repetition rate. While most industrial lasers are limited to 300 Hz repetition rate, the developed laser operates up to 1.5 kHz.
The development of small excimer lasers with relatively small pulse energies but with the capability of reaching high repetition rates, coupled to acceptable gas lifetimes and good beam quality, has opened up new opportunities for laser applications in manufacturing. In some niche applications, such as ophthalmology and micromachining, these lasers provide comparable energy density to their much larger counterparts and with proper overall laser system design, sufficient target areas can be covered. Such small lasers allow production engineers to seriously consider using such laser systems due to the reduced capital investment and small footprint. This paper explores some of the new applications and benefits of this technology.
As applications have evolved out of the research areas, laser beam properties and component lifetimes have become critical to achieving low operating cost in a manufacturing environment. We will discuss the development of a 110 watt KrF laser using an all solid-state pulsed power system. Solid state pulsed power enables a significant reduction in system operating costs by greatly extending the exchange interval of the pulsed power and discharge chamber modules. Beam properties of the laser using both stable and unstable resonator configurations will be discussed.
Design and operating parameters of powerful KrCl((lambda) approximately 222nm), XeCl((lambda) approximately 308nm) excilamps with different discharge geometry pumped by continuous glow discharges along with Ar2((lambda) approximately 126nm) and Kr2((lambda) approximately 146nm) excilamps pumped by barrier discharge are presented. Excilamps with high spatial uniformity of the output and gas lifetime up to 100 hours were developed. It was shown that efficiency of luminescence of exciplex molecules of about 30 percent can be obtained in high-voltage glow discharge and positive column of glow discharge. Output at (lambda) approximately 222nm up to 200 W from single excilamp and 500 W from three excilamps, operated in parallel, was demonstrated.
Polycrystalline-silicon TFT technology is opening the door to highly reliable, high-resolution, high-performance, large AMLCDs that will be inevitable for HDTV and other advanced applications. For formation of polycrystalline silicon, excimer laser annealing has shown to be superior to all other techniques with respect to quality, reliability and economy. In excimer laser annealing a high-power laser beam is scanned over the surface of the substrate, coated with amorphous silicon. The amorphous silicon is heated up within a few nanoseconds, melts and recrystallizes into polycrystalline silicon. The pronounced nonlinearity of the annealing process, the high quality requirements and the high process speeds in production lines make high demands on the laser beam parameters such as energy stability and beam uniformity, and on laser output power. This presentation will discuss the results of recent development in high-power excimer lasers for annealing, and their impact on production of AMLCDs.
A high average power, picosecond pulse XeCl excimer laser system has been demonstrated. By using spatial filtering of the XeCl preamplifier, and by forming pulse bursts of 16 pulses with 2-ns pulse spacing, amplified spontaneous emission has been successfully suppressed in the output of the final amplifier stage. On average, each pulse in the burst is amplified to a level of 15 mJ, corresponding to approximately twice the saturation fluence for XeCl. Peak power in the picosecond pulses is 1.5 GW at 10 ps pulse duration, and the average power of the system is 68 W at 300 Hz burst repetition rate. These pulse bursts have been focused on a copper tape target to generate a laser-produced plasma which emits an x-ray power of 2 W into 2(pi) steradian solid angle, in a spectral band from 9-13 angstrom.
The implementation of 193nm laser lithography for IC manufacturing is partially dependent on establishing energy efficient laser beam delivery systems of 'beamlines' in wafer steppers and other lithography and metrology tools. The objective of this work is to study the parameters that most critically impact 193nm UV energy efficiency, specifically the elimination of ozone from the beampath by providing an inert gas positive pressure ambient around the laser optics, and the use of 193nm optimized mirrors for beam delivery. An experimental 193nm laser beamline was set up with an ozone monitor and several UV detectors used throughout the optical system. 193nm-optimized laser mirrors were tested in comparison with standard laser mirrors. Three different inert gases were introduced and at various pressures while firing the laser at 50 Hz, 100 Hz, and 200 Hz reprates. Laser pulse energies are reported under these varying conditions as a function of ozone concentration. Overall laser beamline energy transmission is given as a function of laser mirror type.
A reduction is achieved in the number of droplets and/or large particles scattered on the surface of grown film deposited by the pulsed laser deposition method. An aperture plate is placed in front of the ablation target in order to block the scattering of droplets toward the substrate. The angle dependence of the number of scattering droplets is investigated, and the optimum aperture size and substrate position are determined. Using an aperture with 5 mm- diameter hole and placing the substrate in an off-axis position result in a significant reduction of the number of droplets. The aperture is found to prevent the scattering of droplets without substantially diminishing the deposition rate of the film.
Pulsed lasers are becoming a popular tool for the deposition of coatings and active layers. A series of thin films composed of binary, tertiary, and quaternary II-VI semiconductors was coated onto silica substrates by the pulsed laser deposition technique, using a xenon chloride excimer laser. The semiconductor films produced by this technique included both films made from a single semiconductor target already containing the desired final stoichiometry and films made by depositing alternating layers from targets of different semiconductor compositions. The optical behaviors of the resultant films were characterized by absorption spectroscopy.
Excimer laser irradiation has been shown to yield unique results when used to process metallic surfaces. We have investigated surface texturing effects on 2024 aluminum alloy specimens. A pulsed XeCl excimer laser was used to generate irradiation fluences ranging from well below to substantially above the ablation threshold. Our results indicate that novel micron-dimensioned surface texturing is achieved over selected fluence regimes. At elevated fluences, texturing is lost due to the extensive surface- melting process. Under certain conditions, sub-micron dimensioned surface structures were generated.
Excimer-laser processing techniques can be extended to a broader and more diverse range of materials by moving to vacuum-ultraviolet laser source such as the molecular fluorine laser. The 157-nm output wavelength takes advantage of the high opacity in most materials and a short pulse duration to minimize thermal loading of target surfaces. The laser readily drives photochemical interactions and affords patterning of approximately 0.1-micrometers features. In this paper, we summarize the recent progress in our laboratory on applying these principles to the development of F2 laser applications. Examples include micromachining of high- bandgap optical materials, fabricating rib waveguides, growing debris-free silica films, driving photosensitivity responses in optical fibers, photochemical processing of III-V semiconductors, and writing fine-feature holographic structures.
The excimer ablation lithography (EAL) is a process of direct patterning and removal of a resist polymer film by photo-decomposition ablation. Comparing to the conventional photolithography, EAL does not need the development process and realizes a non-vacuum dry removal of resist. The main equipment for the new processes is a kind of aligner- exposure for the resist patterning and the removal, which reduce the cost of the clean room and the equipments considerably. This is very attractive for TFT-LCD manufacturing, as it is required to reduce the cost severely. The large area patterning and high throughput are essential for TFT-LCD applications. To prove the feasibility, we fabricated an experimental equipment for ablation patterning. It is equipped with the high precision 300 X 300 mm X-Y stages and a N.A. 0.1 image lens which enable to explore the problems inherent to TFT panel of a real size. In addition, two substantial technologies were developed. One is a dielectric multilayer mask on 8 inch quartz substrate with precision enough for TFT patterns. The other is high ablation rate resist polymer. With these technologies, A4 size TFT laser was fabricated by step and scan method. The results show that EAL is in a good prospect for a new TFT manufacturing technology.
In this paper examinations concerning the laser assisted precision cutting and material removal of Al2O3 based printed boards, structuring of SiC-sealing and bearing rings, and ablation of ferrite ceramics are presented. These investigations were performed with 248 nm excimer laser radiation. The required tolerances of the 2D and 3D geometries are smaller than 2 micrometers . Furthermore, the depth of the heat affected zone should be less than 0,5 micrometers and the crack length not greater than the pore diameter of the ceramics, A specially designed machining station and the necessary processing technology will be presented. Further investigation have dealt with the repair and joining of carbon fiber reinforced plastics. Here again, 248 nm excimer laser radiation was used for selectively ablating the epoxy resin without damaging the fibers. Subsequent material examinations show an increased strength of the repassed samples in comparison to unprocessed material.
Laser ablation of composite materials consisted of elastic polymer and carbon black was carried out for surface modification of composite sheets. The ablated surface was studied by scanning electron microscopy, XPS, and Raman spectroscopy. High intensity irradiation produced cylindrical protuberances on the surface. With the increase of laser pulses the protuberances became large in 20-30 micrometers by aggregation of small ones. Low intensity irradiation produced dome-like Voronoi polyhedrons as the results of development of dispersed carbon particles. The microstructure formation is of interest from the viewpoints of tribological applications.
The excimer laser ablation lithography (EAL) is a process of direct patterning and removal of a resist film by photo- decomposition ablation. Taking advantage of EAL process, we have tried to apply this process to LCD fabrication. As the most fundamental problem for resist materials is the ablation rate, we have examined to measure the ablation rates of many kinds of polymers. AMong them the most promising is the polyurethane (PU) which is synthesized from toluenediisocyanate and poly derivatives in chlorobenzene solution. The ablation rate at 100 mJ/cm2 fluence of 248 nm is more than 0.05 micrometers /shot, which is the highest value of all the materials that we have examined. Through the investigations of structures of PU, we could elucidate the mechanisms of the high ablation rate, and accordingly the molecular design concept of the ablation resist.
We have developed novel photopolymers based on the triazeno chromophore group. The absorption properties can be tailored for a specific irradiation wavelength. With the introduction of a photolabile group into the main chain of the polymer we expected a mechanisms which is mainly photochemical. This should result in high resolution etching with no thermal damage or chemical/physical modification to the material. The gaseous products of the photochemical decomposition were thought to assist the material removal, and to prevent the re-deposition of solid products which would contaminate the surface. We confirmed that the irradiation of the polymer at 308 nm resulted in high resolution etching. No debris has been found around the etched corners. Maximum ablation rates of about 3 (Mu) m/pulse were achieved due to the dynamic absorption behavior. No physical or chemical modifications of the polymer surface could be detected after irradiation at the tailored absorption wavelength, whereas irradiation at different wavelengths resulted in modified surfaces. The etching mechanism can be described as a laser induced microexplosion, revealed by ns-imaging. The etching of the polymer starts and ends with the laser pulse, shown by ns- interferometry, confirming that the acting mechanism is mainly photochemical at high fluences for our polymers, which can be used as high resolution laser dry etching resists.
The excimer laser provides the necessary optical resolution and sufficiently high fluence to permit rapid micro- structure patterning of polymers and glasses by ablation. Micro-scale gratings and structures formed in this way have potential applications in the fields of opto-electronic devices, display technologies and environmental sensors. Conventional broad-band excimer lasers of poor spatial and temporal coherence can be used to write sub-micron gratings with an appropriate silica phase mask in proximity mode. This simple technique has been used to fabricate fiber Bragg gratings and relief gratings on polymers. The proximity of the mask and target increases the likelihood of damage to the mask during ablation. An alternative approach using Talbot re-imaging is attractive as the mask can be remote from the samples and undesirable orders are rejected. We describe the design of a Talbot interferometer in which the zero and first order beams from a grating are recombined and experiments using this with 193 nm ArF laser illumination to form submicron gratings on polymers and in fibers.
The development of new processes for the micro-treatment of material is the basis for increasing integration and miniaturization of mechanical, optical and electronic components. Pulsed high power excimer lasers offer in combination with a micro-machining system, the possibility of manufacturing highly complex components in different materials like ceramics, glass or metals, Because of the increasing number of technical applications, the need for automatic processing has grown in the last few years. While complete working stations are available for Nd:YAG and CO2 lasers, there is a lack for automatic micro-removal with excimer lasers. For complex microstructures like micro- optics, manual programming of the workpiece handling system becomes uneconomic because of the very high number of laser pulses required. Especially for prototypes and small batches where the workpiece geometries change quickly, the development of a universal and automatic machining concept plays a key role for this technology. For this reason, a general machining concept based on excimer laser removal has been realized beginning with the possibility to construct the workpiece geometry by CAD-design tools. A preprocessor allows to calculate the removal volume based on laser specific ablation volumes. The superposition of each laser pulse removal leads to complex 3D surface structures. Moreover, a general movement strategy optimizes the processing speed. For closing the process chain the realized preprocessor automatically generates the necessary NC-data for the implemented CNC-control system. Functionality of this concept has been proven by manufacturing different two and three dimensional micro-structures like micro-optical components.