Fabrication Of Nanometric Aperture Arrays By Wet Anisotropic Etching For Near-Field Optical Memory Application

Ln conventional optical memory, the areal recording density is ultimately limited by diffraction of light since the recorded pit size depends on that of the focused laser beam spot. On the other hand, near-field optical memory currently receives a great attention as a means to increase the recording density drastically without limitation by diffraction since the pit is iiormed by a localized light on the apex of probe. The near-field optical memory of recording iknsity as high as 45 Gb/in2 was demonstrated [l], where tapered optical fiber probe with a tsubwavelength aperture on its end was employed. The near-field optical memory employing the fiber-type probe, however, lacks in high ta transmission rate, as is the case with other high-density memory based on scanning probe technique. This is mainly because it is impossible to scan the probe by a piezoelectric actuator at high enough speed while maintaining the tip-medium separation as close as order of ten nanometers. To overcome this problem, we have proposed a novel apertured probe array [2], as illustrated in Fig. 1, fabricated by Si planar process. It has concave pyramidal shaped grooves with nanometric apertures on their bottom ends. By combining the probe array with near-contact flying head technology of hard disk, e.g., we can realize high read-out rate in ultrahigh density near-field optical memory. In this paper, we describe the fabrication of the probe array by using the micromachining technique of Si. Our special concern is how to establish the fabrication method of nanometric-sized apertures with high reproducibility. The probe array was fabricated by lithography and anisotropic wet etching. As a block layer for further etching, we used the buried oxide of a silicon-on-insulator (SOI) wafer. A wafer with SO1 thickness of 9 pm was thermally oxidized to form 1.5 pm-thick S O , film. Large window regions with several square millimeters area were photolithographically defined the back side of the wafer to remove the sustaining bulk Si. The back side was anisotropically etched with a KOH aqueous solution (10wt.%, 80°C) until the etching stopped to expose the buried oxide layer. The thin upper Si layer on the front side was patterned in a 10 pm X 10 pm square array with photolithography, followed by the anisotropic etching. After the formation of the concave pyramidal grooves which are faceted with (111) planes of Si constrained by the oxide mask, the grooves slowly expand in the (111) direction. The etching was stopped at the instant the buried oxide appeared. The whole oxide was stripped away with BHF to form small apertures in the bottom of grooves, and a gold film was sputter-deposited from the front side to block the far-field light transmission. Finally, edge part of the Si bulk was removed by cutting off. Figure 2 shows the typical scanning electron micrographs (SEM) of the fabricated aperture array. The lateral aperture size of the probe array was 200 nm after 50-nm thick gold film deposition, which was fabricated with high reproducibility. We confirmed that aperture size as small as 80 nm could be obtained by this process. This dimension is below the wavelength of light and orders-of-magnitude smaller than conventional micromachined