EUV lithography is the most promising candidate for the replication of patterns for the 45-nm technology node and below. There are many issues in EUV lithography currently under intensive investigation. They concern (1) a high-power light source, (2) aspherical optics, (3) the exposure system, (4) multilayer coatings, (5) defect-free masks and (6) resist processes. This paper reports on recent research in all these areas except the high power light source. The required output power of the light source is discussed based on a detailed analysis of throughput. However, there are many parameters related to throughput. In this regard, it should be noted that not only a light source with a higher output power but also more sensitive resists and more reflective multilayer mirrors are indispensable if EUV lithography is to become a practical tool.
Historical review of high-rep-rate laser-plasma x-ray source is presented with being focused on approaches using Xe cryogenic targets and related issues. Recent achievements on EUV output power in international competitions and collaborations by research groups are summarized.
Extreme ultraviolet lithography (EUVL) is the main candidate for next generation lithography (NGL) to be introduced at the 45 nm node. The EUV light source requirements are very high, however, demanding an output power of 115 W and an energy stability of ±0.3 % (3 σ, 50 pulses moving average). Currently DPP (discharge-produced-plasma) and LPP (laser-produced-plasma) EUV light sources are considered to be able to fulfill these requirements. This paper presents the current status of the light source development at EUVA and includes a review of worldwide activities.
In the Leading Project of MEXT from 2003, Japan, high average power LD-pumped YAG lasers are being developed with specifications of > 1 J, > 5 kHz, and 1-10-ns pulse. All fiber front end system consists of cw 40-mW Yb doped fiber laser (YbDFL) with a fiber Bragg grating cavity, a pulse shaper of LiNbO3, and three stages of YbDFL amplifiers, delivering 2-mW average power of 1-20 ns pulses in 5-kHz repetition rate. The design and test of 500-W class LD-pumped booster amplifier head is discussed. Some key elements of technology for high-peak and high-average power system are reviewed such as a ten-cm silica laser material, a repetitive work of stimulated Brillouin scattering phase conjugate mirror for 10-kW capability, a ceramic material for Faraday rotator for high-average power level, so on.
We describe properties of laser produced plasmas (LPP) to generate extreme ultra violet (EUV) light for next generation lithography. We briefly present three topics related to the LLP-EUV source; laser intensity dependence of conversion efficiency from laser light to EUV light at 13.5 nm wavelength in 2 % bond width for tin target, present understanding of EUV emission, and related experimental results.
The property of extreme ultraviolet (EUV) generation from clusters irradiated with ultrashort intense lasers was studied. The cluster jet was well characterized by the interferometric method and the measurement of Rayleigh scattered light. Conversion efficiency of over 1.1 %/str was achieved with an Xe cluster jet irradiated by a sub-picosecond KrF laser pulse. This method is considered one of the most attractive methods of generating EUV light under debris free conditions. Based on the dependence of the wavelength and the pulse duration on EUV generation, a sub-picosecond UV pulse was found most suitable. By reducing the temperature of the gas, comparatively large clusters can be obtained, even for Kr. So they can also become an efficient EUV converter.
We developed laser-produced plasma X-ray sources using femtosecond laser pulses at 10 Hz repetition rate in a table-top size to investigate basic mechanism of X-ray emission from laser-matter interactions and its application to an X-ray microscope. In a soft X-ray region, laser-plasma X-ray emission from a solid target achieved an intense flux of photons of the order of 1011 photons/rad per pulse with duration of a few hundreds ps, which is intense enough to make a clear imaging in a short time exposure. As an application of laser-produced plasma X-ray source, we developed soft X-ray imaging microscope operating at the wavelength of 13.9 nm. The microscope consists of a cylindrically ellipsoidal condenser mirror and a Schwarzschild objective mirror with highly-reflective multilayer. We report results of performance tests and biological cell observations of the soft X-ray imaging microscope with a compact laser-produced plasma X-ray source.
Photoelectron spectroscopy gives information on electronic states of materials which control properties of materials and devices. Developments of EUV optics and high brilliant pulsed EUV sources in these days have enabled us to develop a new photoelectron spectroscopy, EUPS. With EUPS, we can achieve sub-μm spatial resolution, which is more than one order of magnitude better than that of a commercial XPS. EUPS can observe one mono-layer surface, and ultra-fast phenomena. In this article, after explaining the principle of EUPS, experimental results on observation of chemical shifts, detection of ultra-low level contamination, observation of laser ablation are described. Studies for improving spatial resolution and energy resolution of EUPS are also described.
A photoelectron spectromicroscope with a monochromatized laser-produced plasma EUV light source has been developed, and its performance was investigated. The monochromatized laser-produced plasma EUV light source can operate at 50 Hz repetition rate. First, various experiments were carried out to investigate the characteristics of this EUV light source. When an aluminum tape-target was used to produce 13 nm radiation, it was observed that there exists an optimal irradiation intensity of the laser beam for excitation. It turns out that the power density ranging from 400 GW/cm2 to 500 GW/cm2 on a target is required. Besides, a spatial distribution of the main spectral line radiation was measured in the cross sectional view to the plasma. It was found that there was a region where the 13 nm radiation is dominant, which exists at the position about 1 mm away from the target surface. Moreover, the absolute radiation intensity of the 13 nm radiation in the EUV light source was measured and estimated to be 4 [mJ/2πsr/±2 %BW/pulse]. Then, by using this developed photoelectron spectromicroscope, photoelectron spectrum observation has been performed. When a GaAs wafer was used as a sample, photoelectron spectra of Ga-3d and As-3d in GaAs were observed. At present, spatial resolution of less than about 5 μm could be obtained. This result shows that the developed photoelectron spectromicroscope can be applied to valence-band electron analysis successfully.
In this paper, the possibility of a new deposition technique named Laser Plasma Assisted Deposition (LPAD) is explained. It is indicated that photoelectrons on the glass surface were excited due to VUV emissions from the double-stream gas-puff target. The excitation remains for a longer decay time than the irradiated laser pulse width. According to the experimental results, the new binding condition would be created with other particle beams from the evaporator during the excited period with the LPAD technique.
GdYCOB is a promising nonlinear optical crystal that possesses characteristics of an effective nonlinear coefficient, noncritical phase matching operation, and high chemical stability. In this paper, we report about fabrication and characteristics of a monolithic wavelength converter, which generates ultraviolet light by the incidence of near infrared laser. The converter consists of GdYCOB for third-harmonic generation, KTP for second-harmonic generation, and a wave plate for polarization-control. These constituents are integrated with an optical contact bonding. The extreme downsizing of ultraviolet generator is possible with this converter as compared to the conventional method. GdYCOB has the advantage of wide angular bandwidth whereas KTP exhibits wide temperature bandwidth. Consequently, the combination of these crystals results in highly efficient and stable ultraviolet conversion. The converter does not necessitate any high-temperature curing or desiccators since the constituent crystals are non-hygroscopic. Thus the monolithic wavelength converter enables a very compact and highly reliable ultraviolet laser.