Mitsuaki Amemiya

Particle-free mask handling techniques and a dual-pod carrier

In EUV lithography, particle-free handling is one of the critical issues because a pellicle is impractical due to its high absorption. To investigate this subject, we have developed a mask protection engineering tool that allows various types of tests to be carried out during the transfer of a mask or blank in air and in vacuum. We measured the number of particle adders during the transfer of a mask blank in a dual-pod carrier and in an RSP200 carrier. We found that the number of particle adders (>=46 nm PSL) to a mask blank in a dual pod is less than 0.01 over the whole process from taking the blank out of the load port in air to putting it in the electrostatic chuck chamber in vacuum. Through various experiments, the number of particle adders during any process using a dual pod was found to be very few and very stable. In contrast, for a naked mask, many particle adders were found in large variations. Below one particle were added in over 80% of experiments on a dual pod and in about 20% of experiments on a naked mask. Based on the test results, we can conclude that the use of dual pod is an excellent particle-free transfer technique.

Phase measurement of reflection of EUV multilayer mirror using EUV standing waves

Projection optics of an EUV lithography system consists of multilayer mirrors. Phase of the incident beam is shifted on reflection at the multilayer mirror [Y. Watanabe et al., Jpn. J. Appl. Phys. 30, 3053 (1991)]. If the phase shift at reflection of a multilayer is not well controlled, it becomes the cause of wavefront aberration. The phase shift depends on the incident angle and the wavelength of the beam. The phase shift is also dependent on the structure of the multilayer. Certain kinds of structural change cause non-negligible variation of the phase shift with very little change of wavelength dependency of reflectivity. Therefore, not only reflectivity measurement but also measurement of phase shift is essential to manufacture multilayer mirrors for projection optics. X-ray standing wave technique has been used to characterize multilayer structure [B. Lai et al., Nucl. Instrum. Methods Phys. Res. A 266, 684 (1988); T. Kawamura and H. Takenaka, J. Appl. Phys. 75, 3806 (1996)]. Intensity of electric fi...

Effect of secondary electron from the substrate in x-ray lithography using harder radiation spectra

Proximity x-ray lithography using harder spectra has been proposed as a next generation lithography technology. The secondary electron from the substrate is one of the issues that should be solved to realize this technology. We investigated the effect of the electron from the silicon substrate on the pattern quality when using the shorter wavelength exposure. As a figure of merit for the effect of the electron from the substrate, we proposed a function R which is defined as the ratio of the energy absorption in the substrate to that in the resist. We confirmed the validity of R by comparing it with a Monte Carlo simulation (SPEED). Several exposure experiments, which corresponded to various R values, were carried out by changing mirror, filter, and mask membrane. We found an adequate range of R where the secondary effect can be ignored from these experiments. Furthermore, we showed that R could effectively be reduced by using an absorption edge of the resist material. Bromine is the best material to suppr...

Development of a novel EUV mask protection engineering tool and mask handling techniques

We have developed a mask protection engineering tool (MPE Tool) that simulates various types of tests during the transfer of a mask or blank in air and in vacuum. We performed mask transfer experiments to investigate particle-free mask handling techniques using the MPE and mask inspection tools. We measured the number of particles accumulated during the transfer of the mask blanks. Less than 0.3 particles were added over a path from a load port (in air) to an ESC chamber (in vacuum) and more than half the particles accumulated appeared during the pumping down and purging steps in the load-lock chamber. Consequently, we consider that pumping down and purging are the most important steps for particle-free mask handling.

Study of electrification of extreme ultraviolet lithography mask and adhesion of particles during electrostatic chucking

One of the critical issues for extreme ultraviolet lithography masks is particle-free mask handling. We report that the number of particle adders on the front side of a mask in a dual pod can be reduced to less than 0.01 particles/cycle (>46-nm polystyrene latex) during the process of starting from the load port to placing an electrostatic chuck (ESC) in vacuum. In addition, we find that chucking the mask on the ESC causes serious issues. One of these issues is whether the masks will be electrically charged by chucking the ESC and whether some particles will be added on the front side. We measure the electric potential of the back and front sides of the mask and examine the particle adders. We find that when the mask is electrically floated, potential on the front side of the mask increases during ESC chucking; when the mask is released from the ESC, it is electrically charged. This electrification causes adhesion of the particles. Our experiments show that to protect the mask from particles, the mask must be grounded throughout the entire process. For electrification, we confirm that a dual-pod system is effective in protecting the mask from particles.

Improvement of resolution in x-ray lithography by reducing secondary electron blur

In x-ray lithography, the energy deposition mechanism of secondary electrons generated in the resist and Si substrate is investigated. By reducing resist thickness, the secondary electron blur is reduced because secondary electrons with high energy exit the resist before depositing energy in the lateral direction. Resolution is improved using a thin Cl-containing resist, especially in wavelengths shorter than the K-shell absorption edge of Cl atoms. The secondary electron blur in the wavelength region from 2.5 to 4.0 A decreases using a 40-nm-thick Cl-containing resist with a bottom layer. By using an x ray in this wavelength region, the 35 nm line-and-space pattern can be resolved at a gap of 10 μm. A higher quality resist pattern can be obtained by increasing the Cl contents in the resist.

Study on counting error in particle inspection

In EUV lithography (EUVL) it is important to protect a mask from the adhesion of particles because it is difficult to use a pellicle. At Selete, we evaluated a dual-pod carrier and reported on its ability to protect a mask from particles. In the evaluation the average number of particles added to the mask during several hundred handling-cycles was 0.4. Therefore, it is very important to precisely count the number of particle adders. However, according to the specification of the inspection tool, the counting error was greater than the average number of particle adders in the evaluation. In addition, it is known that the error increases for particles with a size near the detection limit. In the evaluation, we inspected a mask substrate four times and regarded signals detected multiple times as real particles. We studied the counting error by assuming that the detection probability followed a static statistical fluctuation. We found that the expected value of counting error was represented with the equation by the number of initial particles, particle adders, capture rate, and inspection times. Under our evaluation condition, even if no quasi-particles existed, the counting error by a single inspection was estimated to be approximately 4. However, the counting error by our evaluation (four inspections) was estimated to be approximately 0.05. Therefore, we found that the reliability by multiple inspections was much higher than that by a single inspection and that the number of particles near the detection limit could be found precisely by multiple inspections. * This work was supported by NEDO.

LPP-based reflectometer for characterization of EUV lithography systems

An EUV reflectometer, based on a laser-produced plasma (LPP) light source, has been developed for characterization of EUV lithography systems. The reflectometer consists of the LPP light source, a prefocusing toroidal mirror, a grating monochromator, a polarizer, a beam intensity monitor, a refocusing toroidal mirror and a sample stage. The LPP light source is driven by a Nd:YAG laser; the laser beam is focused onto a copper tape target. A debris mitigation system that uses a rotating shutter was developed. Higher-orders formthe grating monochromator were suppressed to less than 0.2% of incident beam intensity by total reflection of three grazing incidence mirros. In order to compensate for beam intensity instability, a beam intensity monitor using a grating beamsplitter was installed between the refocusing mirror and the sample. Beam intensity instability can be corrected to less than 0.1% by using the beam intensity monitor.

Suppression of secondary electron blur by using Br-containing resists in x-ray lithography

Second generation proximity x-ray lithography (PXL-II), which uses shorter incident x-ray wavelengths and resist materials, is expected to be a candidate for next generation lithography. In the PXL-II technique, the x-ray wavelength absorbed in the resist becomes shorter, but degradation of the pattern quality due to secondary electron blur has not been sufficiently evaluated. In this article, we present our investigation of the secondary electron blur’s suppression in Br-containing resist using a lithographic simulator and a Monte Carlo simulator for electron scattering. By introducing the Br element into resist materials, the electron stopping power improves. In addition, secondary electron blur is suppressed in regions with wavelengths shorter than that of the Br absorption edge. In order to evaluate pattern resolution, the image contrast of the lateral absorbed image in the resist is defined. We found that image contrast is improved in Br-containing resist for the wavelength range down to about 4.5 A,...

Evaluation of Particle Protection Capability of Extreme Ultraviolet Lithography Mask Carrier in Vacuum Transfer by Experiment and Calculation

In extreme ultraviolet lithography (EUVL), particle-free mask handling is a critical issue. We have studied particle protection capability of a dual pod, which consists of an outer pod and an inner pod that holds the mask. Our experiment and calculation demonstrated that a dual pod prevents particle addition in a quality area of 142×142 mm2 at the center of the mask, because particles entering the inner pod stopped near the edges. Our experiments showed 8 particle adders (≥46-nm polystyrene latex particle equivalent) for 1850-cycle transfer tests, and the dual pod satisfied the pre-production tool specification. Our measurement repeatability was 0.0015(σ)/cycle with a binominal distribution; however, we found that the accuracy was insufficient for an estimated specification of a high-volume machine. We discussed the necessary handling cycles and a method to improve repeatability by inspecting the area outside the quality area.