Isao Ochiai

Plasma focus x‐ray source for lithography

A bright and reliable x‐ray source for lithography has been developed using plasma focus. Discharge with constant pressure gas, one of the features of plasma focus, makes the x‐ray source system simple and lengthens lifetime. A fine ceramic insulator made of alumina in place of a conventional Pyrex glass insulator improves system reliability. The system operates for more than 105 discharges without maintenance. The lifetime of the system is ten times longer than that of a conventional plasma focus device. The resolution of a pattern printed by multishot exposure depends not only on the diameter of pinched plasma but also on the variation of source position. A new spherical electrode surrounding the plasma‐focusing space is added to stabilize the location of the spot on the axis by eddy currents which exert the Lorentz force on the plasma. The spot position deviation has become negligibly small as compared with the pinched plasma diameter. The x‐ray source size for neon is 1 mm in diameter and 10 mm in len...

Atomic hydrogen cleaning of GaAs and InP surfaces studied by photoemission spectroscopy

Abstract Cleaning of GaAs and InP surfaces by atomic hydrogen (H∗) beam irradiation was investigated using synchrotron radiation photoemission spectroscopy and Auger electron spectroscopy. The surfaces were irradiated by a H∗ beam generated by a hot tungsten tube at a sample temperature of 360°C. After the H∗-beam irradiation, the GaAs and InP surfaces exhibited a streaky (2 × 1) and (2 × 4) RHEED pattern, respectively. It took a longer irradiation time to clean the InP surface than the GaAs surface. Photoemission spectroscopy revealed that the most persistent oxide in the H∗ cleaning process is Ga 2 O 3 for the GaAs surface, and In(PO 3 ) 3 for the InP surface.

Formation of a thin SiO2 film using synchrotron radiation excited reaction

The possibility of altering an oxygen‐adsorbed Si surface to SiO2 using a synchrotron radiation (SR) excited reaction is evidenced for the first time. Oxygen gas is adsorbed on a clean Si surface, and soft x‐ray is irradiated on it by SR. As a result, H partly terminated on the oxygen‐adsorbed Si surface is eliminated and the surface becomes more SiO2‐like. This is proved by x‐ray photoelectron spectroscopy (XPS) analysis. Photostimulated desorption (PSD) of H+ ions, which are emitted from the surface, is also detected during SR irradiation. The Si—O bond formation model followed by H+ PSD explains this oxidation.

Positive Charge Generation at a SiO2/Si Interface due to Bombardment with Metastable Atoms.

The influence of metastable atoms on a SiO2/Si structure is examined to determine the source of damage in ULSI devices during plasma enhanced processes. Holes were generated at the SiO2 surface by the impact of metastable atoms of rare gases. Holes trapped at the interface formed positive charges, and the density of these positive charges increased with the increasing energy of the metastable atoms. The yields of the positive charge generation were between 0.01 and 0.1, which are on the same order as those caused by vacuum ultraviolet photons, and these values are not negligible. Thus, the influence of metastable atoms must be taken into consideration to control the damage that occurs during plasma enhanced processes.

Low‐temperature synchrotron‐radiation‐excited etching of silicon dioxide with sulfur hexafluoride adsorption

Ultralarge‐scale integration circuits now require innovative microfabrication processes in order to achieve gigabit‐scale integration. One approach is to use soft x rays, because they can give excellent spatial resolution by their short wavelength and high‐reaction selectivity by core‐electron excitation. Synchrotron‐radiation (SR) ‐excited etching of SiO2 and Si is studied from the viewpoints of pattern replicability and analysis of etching selectivity between two layers. Deep‐submicron patterns of SiO2 are formed by cooling the specimen with liquid N2 and adsorbing SF6 reaction gas during SR irradiation. Photon‐stimulated desorption ions from SF6‐adsorbed SiO2 and Si surfaces are first investigated. As a result, ion species such as SiF+n and SO+n, which are etching products from the surface, are obtained only from SiO2, and this selective etching of SiO2 is also investigated by x‐ray photoelectron spectroscopy. In this selective etching mechanism, constraint of Si‐etching by passivation of photofragment...

X-Ray Exposure System With Plasma Source For Microlithography

An x-ray exposure system with a plasma source has been developed to be applied in R & D of a deep-submicron device fabrication process. This system features high accuracy align-ment using imaging optics with high-speed signal processing, precision proximity gap control by means of a wafer surface flattening mechanism, and fine pattern replication by incorporating a plasma focus soft x-ray source. The imaging optics are arranged to be diagonally symmetric with respect to the x-ray exposure axis to perform continuous pattern detection (lateral displacement and gap) directly in an exposure field. A unique wafer chuck capable of flattening a wafer surface to within ±0.5 μm by piezoelectric actuators, supported by a six-axis, micro-motion mechanism, makes it possible to align the wafer to the mask (resolution of 0.01 μm) with a uniform gap (15=11 μm). The plasma focus source which emits x-rays with wavelengths in the range of 10 ~ 14 Å from high-temperature neon plasma using the pinch effect induced by a pulse current, has been newly developed. The pattern replication performance is thoroughly examined, showing an alignment accuracy of within ±0.1 μm (2σ), and a fine pattern replication of 0.3 μm.

Vibration of Beryllium Foil Window Caused by Plasma Particle Bombardment in Plasma Focus X-Ray Source

The effect of plasma particle bombardment on beryllium foil windows was investigated by the displacement measurement of the window foil. It was experimentally verified that the beryllium foil began to vibrate after a shot of plasma focus discharge. It was also verified by the displacement measurement that the particle bombardment caused a thermal expansion of the foil. All these dynamic effects of particle bombardment suggested that fatigue fracture of the foil played an important role in the process of the beryllium foil destruction.

Surface Reaction Induced by Multiply-Charged Ions

The surface reactions induced by an electronic potential energy of multiply-charged ions are investigated. A small electron-beam impact type ion source for production of multiply-charged ions was manufactured for this study. The charge state distribution of multiply-charged ions produced in the ion source greatly depends on the sample gas pressure and the potential barrier for ion trapping. In operation with an electron current of 7 mA at 2 keV, the ion-beam currents obtained for Ar5+ and Ar10+ are respectively at least 7 pA and 2× 10-3 pA. H+ ion desorption from a lightly etched GaAs surface are observed by bombarding with Arq+ ions (q=1–3). The desorption yield of H+ per Arq+ increases proportionally with the sixth power of the charge state of Arq+. This charge state effect is discussed from the viewpoint of the electronic interaction between the GaAs surface and Arq+ ions.

Photon-stimulated desorption from chemically treated Si surfaces

Abstract Experimental results are presented on the synchrotron-radiation-induced desorption of oxygen ions from HF-treated Si surfaces measured with a quadrupole mass analyzer operated in the pulse-counting mode. The effect of atomic hydrogen exposure was studied. Desorption of H + , O + and F + ions was observed from HF-treated Si surfaces. The yield of O + ions was increased more than 30-fold by exposure to atomic hydrogen.

A Novel Method of Beryllium Window Protection in a Plasma Focus Soft X-Ray Source

A significant problem of plasma X-ray sources such as a plasma focus device is how to protect the very thin beryllium window from damage and contamination caused by the bombardment of plasma particles. Improvement of the protection of the beryllium window has been made by inserting a magnetic deflector between the focused plasma and the window and by inverting the polarity of the applied voltage to the electrodes. As a result, a 13-µm-thick beryllium membrane can be used safely. The measured X-ray intensity is about 5 mW/cm2 30 cm from the source operated at 3.5 kJ discharge energy. The achieved intensity is suitable for R&D application in X-ray lithography.