Rainer Paetzel

KrF excimer laser with repetition rates of 1 kHz for DUV lithography

Advanced DUV lithography tools are adapting scanning methods in order to match the increasing demands on throughput, field size, and resolution. The demands on the laser source are changing for both the reflective and the refractive Step&Scan tools. The cost-effective operation of such lithography tool requires 248 nm excimer laser with high power and repetition rates of >= 1 kHz. Precise dose control of the exposure demands not only high repetition rates but also excellent stability of the laser output energy. The combination of the new NovaTube technology laser tube design with the LITHO line narrowing optics design allows 1 kHz operation of the excimer laser with high stability.

Excimer lasers for material ablation cross the 1.5 kHz mark

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.

Recent developments of industrial excimer laser technology

The paper reviews recent developments in high power excimer laser technology driven by industrial requirements. Technological achievements as NovaTubeTM laser tube technology and HaloSafeTM halogen generator technology are discussed. Experimental results are presented for various lasers at the most important excimer wavelengths 351 nm (XeF), 308 nm (XeCl), 248 nm (KrF), 193 nm (ArF) and 157 nm (F2) which have been designed for application in micromachining, thin-film-transistor annealing, marking as well as lithography.

ArF excimer laser for 193-nm lithography

The ArF excimer laser light source will extend the optical lithography to below 0.18 micrometers design rules. Still under discussion is the most effective layout of the stepper or scanner imaging optics. The decision for an all-fused-silica lens, an achromatic lens using CaF2, or a catadioptric imaging system has great impact on the laser-bandwidth requirement. In addition, the potential performance and perspective of the laser must be considered in the system layout of the production stepper or step and scan tool. Cost effective operation of such a lithography tool requires a 193 nm excimer laser with high power and repetition rates in the order of 1 kHz or higher. Precise dose control of the exposure demands high repetition rate and excellent stability of the laser output energy. We have developed an ArF laser which can be operated with up to 800 Hz repetition rate based on NovaTubeTM technology. Optimized materials and discharge configurations have been used to achieve laser tube lifetimes above 109 pulses. Up to 4 X 109 pulses tube lifetime have been achieved in beta-site tests. Gas lifetime of several 107 laser pulses is obtained. A solid-state switch has been adapted for the reliable and cost efficient excitation of the 193 nm lithography laser. To achieve laser output of different bandwidths various resonator arrangements have been investigated. The paper gives an overview of the currently achievable power levels at different bandwidths and discusses future trends.

High-repetition-rate lasers for advanced DUV exposure tools

KrF lasers with up to 500 Hz repetition rate and a bandwidth of about 1 pm are in use for DUV-microlithography. Increasing resist sensitivity demands even higher repetition rate in order to allow precise dose control. In some cases step & scan exposure tools apply reflective optics instead of refractive ones. This diminishes the bandwidth requirements by about two orders of magnitude, but a high polarization degree of >= 98% is a basic requirement for the laser light source. Dose control and statistics define the second basic requirement for the laser. A dose accuracy of less than 2% demands small energy increments, i.e., for pulse energy in the 10 to 20 mJ range. Throughput requires 10 to 20 W of average laser power. Therefore, the repetition rate must be in the 1 kHz range.

Excimer lasers in manufacturing

The year 1997 will become known as the year of commercial excimer lasers. The number of installations on manufacturing and medical floors will be much higher than ever before. Besides the medical uses the most important applications are DUV lithography, TFT annealing for flat panel displays and microdrilling for nozzle arrays, especially for ink jet printer heads. Recently two very important breakthroughs were achieved. The first is in regard to the performance of the excimer laser itself. The second is a much more efficient way of using the UV photons. These are the main reasons for the actual success. Technology development by laser manufacturers has resulted in remarkable improvements of component lifetime, reliability, cost of ownership and ease of use. With gas and optics lifetimes in excess of 108 pulses, laser tube exchange intervals longer than 5 multiplied by 109 pulses and integration of internal halogen generators a quasi sealed-off excimer laser with hands-free operation is accomplished. The new generation NovaLineTM combines the experience of more than 4000 installed excimer lasers with a completely new laser engineering design. Progress in advanced UV optics will be demonstrated with two examples. In commercial production of very precise nozzle arrays, high-end optics allow the drilling of all nozzles simultaneously with an optical distortion of less than 0.5 micrometer over a full 0 18 mm processing field. For scanning applications as AMLCD annealing, the rectangular laser beam can be optically transformed into a very homogeneous line, up to 300 mm long and 0.5 mm wide.

Extremely narrow-bandwidth high-repetition-rate laser for high-NA step-and-scan tools

In this paper we discuss several resonator designs in terms of bandwidth, efficiency and lifetime of the major optical components. Experimental data are presented for a resonator combining the advantages of a long lifetime grating and etalon elements. Moreover, the limitations of these elements are overcome through the outcoupling scheme, ensuring optimum feedback over gas and tube lifetimes. In addition to low cost-of-ownership, this same resonator provides for an extremely narrow bandwidth ArF excimer laser development, using a single oscillator.

Excimer laser: innovation in industrial material processing

Excimer lasers offer unique benefits for a wide range of applications, including industrial materials processing, scientific research and medicine. The benefits of the excimer laser stem from its high peak power output delivered in short pulses at a variety of UV wavelengths. In recent years, technical developments by laser manufacturers have lead to remarkable improvements in excimer performance, reliability and utility, as well as lower cost of ownership. As a result, the market for excimer lasers continues to grow and diversify. In this paper we examine some of the more recent advances, look at various industrial applications that are enabled by excimer lasers, and catch a glimpse of the future direction of this technology.

NovaTube: first excimer laser design for quasi-sealed-off operation

Excimer lasers are the most efficient and powerful sources for ultraviolet radiation. Our new laser cavity design NovaTubeTM is the result of many years of extensive material research. Several test runs with NovaTubeTM lasers at 193 nm (ArF) and 157 nm (F2) demonstrated excellent gas lifetime data when compared to conventionally designed lasers. For the first time a 50 W KrF laser was successfully operated sealed-off for more than 1 billion pulses. By this outstanding performance the operating costs can be cut 10 times using the new laser tube technology.

NovaTubeTM: a breakthrough toward sealed-off excimer lasers

NovaTubeTM is the result of many years of extensive material research at Lambda Physik to develop a laser cavity of quasi sealed-off laser performance. As test data demonstrate the operating costs can be cut 10 times using the new tube. Test runs in the laboratory have culminated in 109 shots operation on a single gas fill achieved with a 50 W KrF laser.