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Micro/Nano Lithography

Vacuum UV applications for nanoscale processing

Vacuum-UV radiation is useful for surface microanalysis and has enabled development of a new, high-quality chemical vapor-deposition technique operating near room temperature.
11 February 2010, SPIE Newsroom. DOI: 10.1117/2.1201001.002554

Current methods of semiconductor fabrication, such as deposition and cleaning, often use catalysts and solvents to induce desorption, chemical reactions, and decomposition. However, many catalysts and solvents are poisonous and must be rendered harmless. In addition, high temperatures are preferred for fabrication of high-quality products. For surface-analysis techniques specifically, thermal decomposition spectroscopy (TDS) can be used to detect trace amounts of inorganic impurities. TDS detects desorbed gas from heated samples through mass analysis. Unfortunately, however, deterioration of substrates at high temperatures often becomes a problem.

Short-wavelength radiation is in high demand for novel, precise materials processing. Vacuum-UV (VUV) emission from excited-dimmer (excimer) lasers is very attractive for nanoscale approaches.1 VUV photons are strongly absorbed by most materials and may trigger previously unknown phenomena, including efficient photochemical reactions.

The writing pattern in semiconductor structures becomes narrower at nanometer scales. The insulation layer should fill the gap between conducting layers with a high aspect ratio. High-temperature processes induce substrate deterioration, which in turn reduces carrier mobility. This results in a slow processing speed. Chemical vapor deposition using VUV lasers (VUV-CVD)2,3 represents a promising candidate process at low temperature for film deposition characterized by low substrate damage. To deposit silicon dioxide (silica film), VUV photons irradiate the primary substrate, such as tetraethyl orthosilicate or tetramethyl cyclo-tetrasiloxanes, and trigger decomposition (see Figure 1). The main material components include a Si–O (silicon–oxide) structure. In optimum conditions, the ambient gas provides the silica films with a good filling factor and self-smoothed surfaces. We have demonstrated gap-filling deposition with an aspect ratio in excess of 10 (see Figure 2). VUV-CVD can also deposit silica film onto organic substrates, because the deposition was achieved near room temperature.

Figure 1. Concept of the silica-deposition process by chemical vapor deposition with a vacuum-UV (VUV-CVD) excimer lamp and tetraethyl orthosilicate (TEOS).

Figure 2. Cross-section of a film deposited by VUV-CVD.

VUV photons induce desorption as well. When we irradiated the organic materials adsorbed on the substrates with VUV photons, they strongly absorbed the photons and subsequently desorbed them from the surface. We analyzed the desorbed gas with a TDS-like mass spectrometer using photo-stimulated desorption spectroscopy (PDS). The desorbed gas was decomposed into fragments. Even if the fragment masses were the same, the signal intensities from the different materials exhibited different time dependencies.4 One can identify the contaminating material by measuring the time variation of the mass-signal intensity. For our VUV-PDS method to work efficiently, tunable VUV radiation is preferred. We therefore developed a new laser-plasma VUV source. We obtained VUV emission from a rare-gas plasma when the incident neodymium-doped yttrium-aluminum-garnet laser (with a wavelength of 1064nm and an intensity of 1011W/cm2)5 was focused into a range of gases at 1atm (see Figure 3). We measured broad VUV emission from the argon-gas plasma, suitable for use in our tunable VUV source.

Figure 3. VUV emission spectra in arbitrary (arb.) units from rare-gas plasmas: helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe). N I: Neutral nitrogen. II, III: Single, double excitation.

VUV photons have sufficient energy to excite and resolve the sample materials. These photons can induce novel reactions and enable new material developments. In addition, VUV-photon processing needs neither catalysts nor solvents, and can perform at low temperatures. From the perspectives of both manufacturing costs and environmental concerns, coherent VUV sources show promising industrial applicability. We are currently developing a new VUV-laser system for high-intensity output with high repetition rates, using a novel optically pumped excimer medium.1

We acknowledge contributions by and discussions with many collaborators, in particular M. Kaku and S. Kubodera. Part of this work was supported by the Japanese Ministries of Education, Culture, Sports, Science, and Technology, and of Economy, Trade, and Industry. Our PDS system is commercially available in Japan from both Network Technology Partners (NTP) Inc. and ESCO Ltd.

Masahito Katto
University of Miyazaki
Miyazaki, Japan

Masahito Katto is an associate professor. He received his PhD in engineering from Osaka Prefecture University. His research focuses on developing short-wavelength lasers and their applications.

Atushi Yokotani
Department of Electrical and Electronic Engineering
University of Miyazaki
Miyazaki, Japan

Atushi Yokotani is a professor. He received his PhD in engineering from Osaka University. His research focuses on applications of lasers and short-wavelength radiation.