Ikuo Okada

A plasma x‐ray source for x‐ray lithography

Employing a gas‐‘‘puff’’ z‐pinch plasma, a high brightness and compact plasma x‐ray source has been constructed by using a high‐repetition‐rate discharge and a new x‐ray extraction method. The high‐repetition‐rate discharge (3 Hz) is attained by using a fast‐acting gas valve, capable of operating stably under gas plenum pressures below 300 Torr. This low gas pressure discharge makes possible high‐conversion efficiency of electrical input to x‐rays, highly reliable plasma pinching, and a drastic reduction of debris resulting from electrode vaporization. An axial x‐ray exposure is required to reduce the blur in pattern replication caused by the penumbral effect. To carry out the axial exposure, the high‐energy particles emitted from the plasma are removed using a plasma reflector plate and magnet. The x rays are efficiently extracted through a window highly transparent to x rays. With this x‐ray source, 9–14 A x rays with 200 J per pulse (600 W average power) are obtained using Ne gas plasma in a 3 Hz opera...

Mask Contamination Induced by X-Ray Exposure

To investigate the growth of mask contamination induced by X-ray exposure, masks are exposed to X-rays using an X-ray stepper. Independent of the exposure conditions, and in the presence or absence of resist coating, the main component of the contaminant is the salt (NH4)2SO4. On some resist-coated wafers, the contaminant growth is governed by the volume of sulfuric materials which evaporate during exposure. Contamination growth can be suppressed by applying a top coat to the resist, as using a resist that does not evaporate as a sulfuric out-gas. Gases were irradiated by X-rays in order to examine the change in environmental pollutants. In humid air, NO, NO2 and NH3 form rapidly up on X-ray irradiation. SO2 also increases up on X-ray irradiation, and changes into H2SO4. H2SO4 easily combines with NH3 which is produced by X-ray irradiation in humid air, and turns into the salt (NH4)2SO4. Mask coating was introduced to suppress the contaminant growth. The mask coated with TiO2 layer does not show contaminant growth, showing a drastic curtailment of contamination by the photocatalysis effect.

Synthesis of metallic nanoparticles through X-ray radiolysis using synchrotron radiation

The potential to fabricate metallic nanoparticles directly on silicon substrates from liquid solutions is ideal for three-dimensional lithography systems, drug delivery materials, and sensing applications. Here, we report the successful synthesis of Au, Cu, and Fe nanoparticles from the corresponding liquid solutions [gold(I) trisodium disulphite, copper(II) sulfate, and potassium ferricyanide] by synchrotron (SR) X-ray irradiation. The deposition of gold nanoparticles in the gold(I) trisodium disulphite solution was performed by monochromatic X-ray exposure from synchrotron radiation. The use of ethanol as an additive enabled the nucleation and growth of Cu particles, while no Cu particles were produced in the copper sulfate solution without ethanol with polychromatic SR X-ray irradiation. Fe particles were generated by direct polychromatic SR X-ray irradiation. These results demonstrate the behavior of three-dimensional printers, enabling us to build composite material structures with metallic and plastic materials.

Performance Test and Evaluation of Multilevel Fresnel Zone Plate with Three-Step Profile Fabricated with Electron-Beam Lithography

A multilevel Fresnel zone plate (FZP) designed as an approximation of a kinoform profile has been developed. The FZP is made by electron-beam lithography and reactive ion etching. The zone structure consists of three levels made from tantalum with a total thickness of 4 µm. The half pitch of the outermost zone structure is 0.6 µm. The theoretical efficiency of the 1st-order diffraction is 0.49 at an X-ray energy of 9.85 keV. A performance test is carried out at the beamline 20XU of SPring-8. The focused beam width is measured to be 0.6 µm. The measured efficiency of the 1st-order diffraction is 0.39 at 9.85 keV. Although the measured efficiency is less than the theoretical value, it is superior to the efficiency of an optimized tantalum binary FZP. The diffraction efficiencies of the 0th, -1st, and ±2nd orders are also measured. Using the measured data, the structure of the fabricated zone is evaluated.

Development of highly accurate X-ray mask with high-density patterns

We have previously developed X-ray masks using SiN film as a mask membrane and Ta film, deposited by ECR sputtering, as a mask absorber. However, it is necessary to improve the pattern placement accuracy of X-ray masks for practical fine pattern lithography. In this study, we newly employed (1) a membrane process, (2) SiC film as a membrane, (3) Ru film as an intermediate film between the membrane and the absorber, and (4) a distortion compensation method called PAT by EB writing to reduce the pattern placement distortion. By combining these techniques, highly accurate X-ray masks with high-density patterns, which correspond to 4 Gbit class DRAM patterns, could be fabricated with the distortion of less than 40 nm.

Progress in x-ray mask technology at NTT

This article overviews NTT’s recent progress in membrane and absorber materials for x-ray masks and the fabrication of highly accurate masks for synchrotron radiation lithography. As membrane materials, SiC and diamond films are being studied. Experimental results indicate that diamond films are better than SiC from a mask accuracy point of view because of their higher rigidity. As absorber films, Ta films deposited by electron cyclotron resonance (ECR) sputtering are studied. The Ta films have the α structure and large grains, and are chemically very stable. All masks are produced by a membrane process in which the patterning of absorber films is done after backetching. The absorbers are patterned by a new electron beam (EB) writer with an acceleration voltage of 100 kV and ECR ion stream etching with He gas cooling. Ta patterns with widths below 100 nm are etched accurately. Placement accuracy of absorber patterns produced by this process is confirmed to be about 30 nm, using relatively simple patterns....

Fabrication of x-ray mask from a diamond membrane and its evaluation

We have prepared a diamond membrane x-ray mask using our novel fabrication techniques which include fluidized bed pretreatment, stress control of diamond film, and chemical-mechanical polishing (CMP), and we have evaluated its important properties for practical use in detail. Surface roughness of diamond film was improved from 55.7 to 1.5 nm Ra using the CMP technique. The transmittance of visible light increased considerably due to the reduction of incident light scattering at the diamond surface, and the transmittance of 2.2-μm-thick smooth diamond film was 92% at 633 nm. In-plane distortion-induced synchrotron radiation irradiation was Max(x,y)=(13,10) nm with a dosage of 12.8 kJ/cm2. In final evaluation, that is, of lithographic performance, we confirmed that a 0.2 μm gate array pattern can be successfully transferred.

Defect‐free x‐ray masks for 0.2‐μm large‐scale integrated circuits

The x‐ray mask fabrication process we developed can be used to make and maintain essentially defect‐free masks consisting of Ta absorbers on SiN membrane. The surface of the deposited SiN substrates is polished to make them as smooth as possible. As Ta is chemically stable, mask fabrication processes are frequently followed with a wet‐cleaning process using a strong acid solution. Inspection of resist patterns printed on a wafer confirms that there are less than 20 printable defects per mask, and there are no defects on the back surface. The inspection and repair are repeated until the mask is defect free. Defects caused by mask handling and x‐ray exposure are immediately washed up by wet cleaning with strong acid. The resulting x‐ray masks have been used in large‐scale integrated circuits fabrication, and fully functional devices have been obtained.

X-Ray Mask Inspection Using Replicated Resist Patterns

A new X-ray mask inspection method using replicated resist patterns is proposed. It is able to detect fatal opaque defects on the back surface of the mask and at the bottom of the hole pattern, in addition to those on the front surface. It can also ignore transparent defects on the mask. This method is useful even for defect detection on a single-die mask through die-to-die comparison. For the false process defects occurring during the replication process, a discrimination procedure using a 2-step die-to-die comparison is proposed. In inspection tests with SEMSpec, we investigate the relation between the detection sensitivity to small resist defects and the conductive-coating thickness on them.

Characteristics Of A Gas-Puff Z-Pinch Plasma X-Ray Source

A high brightness plasma x-ray source is developed. This source can be linked to a conventional aligner by directing x-rays downwards into the atmosphere. By adopting the low pressure gas injection method, a wide discharge timing margin and strong x-ray emission are attained. X-rays having line spectra of 9-14 Å in wavelength are emitted from the Neon gas plasma. As high mask contrast can be obtained, submicron resist patterns can easily be replicated by using a 0.4 μm thick Ta absorber. The mask distortion due to mask heating is not a serious problem in He atmospheric exposure. The exposure time is about 13 seconds with a 3 Hz discharge repetition rate. Compared with a conventional source, a throughput 10 times greater is achieved.