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Recent developments and future aspects of nuclear medicine in oncology were reviewed. As developing fields of nuclear oncology, three topics were reviewed. The first topic is the establishment of human radiation oncology, which includes the establishment of radiotherapy based on the tumor growth kinetics and radiosensitivity of each tumor. Radiopharmaceuticals were introduced for the visualization and quantification of DNA synthesis, hypoxic cells and apoptosis. The second topic is pharmacokinetic study for cancer therapy using specific drugs. For instance, boron neutron capture therapy (BNCT) for malignant melanoma using 10 B-paraboronophenylalanine was introduced. The radiation dose for tumor and normal tissues largely depends on the 10 B-concentration in the tissues; however, it is difficult to measure it directly. 18 F-BPA was developed for visualizing tumor localization and could be used for quantifying the 10 B-concentration of the tumor using positron emission tomography (PET). Gene expression imaging, the third topic, can be performed by two approaches. The first approach is receptor-based imaging. Using a virus vector, a receptor gene, such as the somatostatin receptor, can be transferred to the cells. Then, imaging can be performed using radiolabeled receptor ligands. The second approach is enzyme-based imaging. Gene transfer of herpes simplex thymidine kinase (HSV-TK) into cells and imaging with 124 I-FIAU was introduced.

This paper discusses the application of iterative phase retrieval method to phase measurement in optical lithography. A simple algorithm was developed for reconstructing phase information from image intensity distributions measured under different defocus conditions. The phase distribution in the image plane and in the pupil plane are reconstructed from the images obtained using the micro-lithography simulation microscope, MSM100 (Zeiss). Two applications of this method in optical lithography meteorology were investigated. First, the phase transmission of phase-shifting masks was successfully measured both for alternating and for attenuating type masks. The reconstructed phase showed that the uniform background of light leaking from the opaque regions on the mask can affect the phase in the image plane. Second, the possibility of using this method in measuring aberrations of projection optics was examined.

Phase-shifting masks and imaging characteristics are discussed and compared with those of conventional transmission masks. Then, new phase-shifting masks with intermediate values of phase shifting or transmittance are proposed, and their imaging characteristics are investigated. A phase-shifting mask with a 90° phase difference can ease the restrictions on pattern geometries used in phase-shifting technology but does not increase the focus latitude. It is also suggested that a halftone phase-shifting mask is suitable for printing isolated patterns and it gives wider focus latitude than conventional mask technology.

Phase and amplitude modulation in a mask can achieve amplitude superposition for multiple images along the light axis and phase control between them. This improves the depth of focus while keeping high resolution under high coherence illumination in optical lithography. This method explains the mechanism of the depth-of-focus enhancement effect in "peripherally added" or "edge-enhancing" phase-shifting masks and provides the most effective phase-shifter arrangement. Because a high coherency is required in this method, however, the proximity effect becomes a serious problem in practical LSI patterns.

Several spin-on-glass (SOG) materials were examined as single layer resists for ArF excimer laser lithography, with the goal of directly forming a hard mask from these materials for dry-etching underlying metal films. Perhydro-silazane (PHSN) was found to be photo-reactive at 193 nm wavelength as well as polyphenylmethyl-silsesquioxane (PMSQ) and polyhydroxybenzyl-silsesquioxane (HSQ), which we have reported previously. These materials showed a sufficient resolution performance and sensitivity at 193 nm. The Fourier-transform infrared (FTIR) and X-ray photoelectron spectrometry (XPS) analyses showed that the basic reaction is photo-oxidation, though the imaging mechanism in each material is quite different. The etching resistance of these materials was significantly improved by special treatment after patterning, whereas those without the treatment were insufficient. For example, etching rate of PHSN after baking in steam ambient was comparable to that for CVD SiO2in RIE using SF6gas. 0.2 µ m patterns were transferred into poly-Si films by dry-etching using these materials as hard masks.

Recently, we have investigated the effects of radially non-symmetric pupil filters and multiple-pupil exposure sequences in optical lithography [to appear in: Proc. SPIE 2726 (1996)]. Based on numerical simulations we predicted that extremely narrow lines could be delineated using the superposition of exposures with symmetric and anti-symmetric pupil functions. Here, we present experimental results verifying this prediction. Using conventional masks and a pupil filtering i-line stepper with a numerical aperture of 0.4, we were able to achieve linewidths of 0.18 µm in 1.0 µm thick positive photoresist, with a depth of focus of 1 µm.

Several super-resolution photolithography techniques are investigated. In annular illumination, both the resolution and depth of focus (DOF) are enhanced most effectively when 0.6