Super-resolution Patterning Technique Using Inorganic Material

Nanometer-scale pattern whose size is less than the resolution limit of laser beam can be fabricated using thermal change of an inorganic material.

Lithography is a patterning technique using photosensitive material whose pattern size is the same as the processing laser beam spot size. In the case of heat-mode lithography, which is based on the manufacturing technique for optical discs, the pattern size of less than the laser beam spot size can be fabricated by using an inorganic patterning material. When a focused laser beam is irradiated onto the inorganic material, thermal change of the material occurs only at the center of the laser beam spot owing to the temperature distribution in the laser beam spot (Figure1). We focused on an inorganic thin film (ZnS-SiO₂) that has been used as a protective layer for rewritable DVD. Steep and clear pattern edges are fabricated by using ZnS-SiO₂ thin film as the patterning material (Figure 2). Nanometer-scale patterns are fabricated by a simple process involving only laser irradiation and wet etching (Figure 3).

The pattern size is controlled by changing laser irradiation conditions, and the size is reduced to approximately one-fourth the size of the laser beam spot (Figure 4). In addition, quartz patterns are fabricated by skilful combination of the heat-mode lithography and a dry etching process (Figure 5). The quartz patterns are used as master molds for nanoimprinting and other replication processes. A replication process is required for reduction of mass-production costs of various devices.

In the case of the conventional lithography technique, the pattern size is the same as the processing laser beam spot size. Consequently, it is necessary to reduce the wavelength of the laser beam to reduce the pattern size. Expensive lithography equipment using e-beam or x-ray light sources is used to fabricate a pattern whose size is less than 100nm. In the case of heat-mode lithography, an inexpensive laser irradiation setup can be used because it is possible to fabricate the pattern size less than the laser beam spot size (Figure 6). Furthermore, using a high-speed rotating sample stage in the laser irradiation setup, large-area and high-speed laser irradiation is possible.

Optical Disc

Phase transition mastering (PTM) using an inorganic patterning material is employed as the mastering method for Blu-ray and next-generation optical discs. ZnS-SiO₂thin film is useful patterning material for fabrication of pits and grooves of optical discs, because the pattern shape of ZnS-SiO₂ is clear and fine.

Functional Device

A patterned surface with period shorter than the wavelength of visible light suppresses the reflection of the light (Figure 7) . In the case of functional devices, such as optical elements, light source devices and optical sensing devices, the device characteristics are improved by suppressing the reflection of visible light. A less costly anti-reflection structure is required, because a conventional anti-reflection dielectric multi-layer thin-film coating is expensive. Heat-mode lithography is an inexpensive, large-area patterning process. In view of its advantages, heat-mode lithography is expected to be applied for the fabrication process of anti-reflection structures for light source devices and optical sensing devices.