Near-field optical probe uses VCSEL light source

OPTICAL DATA STORAGE

Data storage, or more precisely the burgeoning appetite of computer users for higher data storage density and recording rates, could soon become the bane of the information technology (IT) community. Demand is already showing signs of outpacing the capabilities of conventional magnetic recording methods. While research based on a parallel arrangement of scanning force microscopy cantilevers shows promise, existing magneto-optical recording techniques are still faster. Near-field optical techniques based on an array of vertical cavity surface emitting lasers (VCSELs) could someday offer data storage densities of Terabits per inch and recording rates approaching 10 Gbit/s per VCSEL. This technique, too, has a catchnamely the strong dependence of near-field optical recording on the nanoscale distance between the read/write sliding head and the storage medium.

In the image of a 15-nm thin aluminum edge on a glass substrate (a), the line marks the location of the profile shown in the intensity profile (b). Working with the profile, the Kassel researchers determined the experimental setup with the VCSEL probe produced a lateral edge resolution of about 100 nm.Click here to enlarge image

Because of such limitations, conventional data storage and recording techniques face little competition from methods based on either scanning-force microscopy or scanning near-field optical microscopybut perhaps not for long. Egbert Oesterschulze and colleagues at the University of Kassel's Institute of Technical Physics (Kassel, Germany) have combined the two techniques. A key component of their experimental setup is a gallium-arsenide (GaAs) cantilever probe that integrates a VCSEL to illuminate an aperture in the metal-coated probe tip. In preliminary tests, the device, which the scientists believe could be easily extended into an array arrangement, scanned a Fischer projection sample (triangular 15-nm thin aluminum islands with 200-nm side length) with an edge resolution of about 80 nm and a contrast approaching 10%.

In the active-emitting near-field probe, the Kassel scientists used thin metal layers of gold/zinc/gold (Au/Zn/Au) and gold-germanium/chromium (Au-Ge/Cr) to electrically contact the p and n-doped layer of the VCSEL.1 The upper layer is electrically insulated from the n-doped GaAs substrate by a silicon-nitride film. The lower layer has a miniaturized aperture at the tip apex. The aperture serves as a near-field source, and is illuminated by the VCSEL, which is a strained layer indium-gallium-arsenide quantum-well setup with an 8-µm diameter and 980-nm wavelength. The probe tip has a pyramidal shape with four {110} sidewalls. Except at the apex, the tip is thermally coated with 200-nm Au-Ge alloy. A thin Cr adhesion layer serves as an ohmic contact for the laser, as well as opaque coating. To obtain the aperture, scientists arranged the tip axis at a 45° angle to the crucible plane of the evaporator. During coating, the tip rotates around its axis, leaving the tip apex almost completely uncoated.

Microscope configuration
According to Oesterschulze, the setup of the near-field microscope is based on an inverse light microscope operated in the transmission mode. A printed circuit board with three parallel copper electrodes holds the probe and also serves to contact the near-field probe. The center electrode serves as the p contact, and the outer conduction paths are ground electrodes connected with aluminum wires to the cantilever holder. The printed board is glued probe-face down to the end of a piezo tube incorporated in a conventional scanning-force microscope. This positioning method allows precision adjustment of the cantilever tip with respect to the optical axis of the inverse microscope.

The researchers control distance in the scanning-force microscopy mode with a beam deflection method. An optical filter with an absorption edge at 885 nm minimizes scattered light in the optical beam path of the microscope, and an avalanche photodiode operated in Geiger mode is used to detect the infrared light of the VCSEL. Adjustment of probe and sample is facilitated with the use of a charge-coupled-device (CCD) camera.

One experiment with the probe determined the aperture size of the VCSEL probe as it scanned across the edge of a 15-nm thin evaporated aluminum film on a glass substrate (see figure). The resulting optical image had a scan range of 6 x 6 µm² and 512 x 512 data points. The image revealed a contrast of about 30%. With this result and the image generated of the Fischer projection sample, Oesterschulze and colleagues believe they have confirmed the feasibility of the aperture probe concept. Equally important, their experiments also illustrated that both the lateral resolution and the large bandwidth of the VCSEL diodes make them suitable for high-density optical near-field data storage in a parallel arrangement.

Paula Noaker Powell

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