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Precision Optical Tuning Technique Using Nanometer Near-Field Laser Ablation

IP.com Disclosure Number: IPCOM000127622D
Publication Date: 2005-Sep-06
Document File: 2 page(s) / 319K

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Abstract

We have been developing a combined technique of using ultra-fast laser in conjunction with fiber-probe delivery of a near-field scanning optical microscope(NSOM) to ablate substrate surface in near-field region. Related technologies can be found in literature. Our technology is able to fabricate sub-wavelength features in ambient condition with nanometer precision. We use 150 femto-second pulses at wavelength of 387-nm. There are two essential advances. To improve sub-wavelength machining, we use a small probe with 150-nm apex to deliver the laser pulses. The transmitted laser beam size and beam cross section are determined by the geometry of the nanometer probe itself. Therefore, under such condition, the diffraction limit is no longer applicable, and the laser beam size can be smaller than half of the wavelength. In fact, Ohtsu et al had shown possible resolution of one-eigth of wavelength. The distance between fiber probe apex and the substrate surface is controlled to be much less than wavelength of 387 nm. Otherwise, laser beam would resume its free space characteristics after it propagates longer distance. To be able to precisely position the probe at the targeted location, we use the atomic force microscopy (AFM) feature of NSOM to track the sub-diffraction limit preformed features on the substrate. This strategy allows us to measure the local topology and deliver the ultra-fast laser pulses using the same fiber probe. This AFM function also enables us to control the distance between the probe and substrate very precisely. The ability of monitoring surface topology of substrate in real time enables us to accomplish the in-situ surface processing. In other words, we are able to do nanometer scale repair/adjustment on a preformed nanometer structure, which is difficult or impossible by other technique, for example etching.

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Precision Optical Tuning Technique Using Nanometer Near-Field Laser Ablation

   1. Introduction: We have been developing a combined technique of using ultra-fast laser in conjunction with fiber-probe delivery of a near-field scanning optical microscope(NSOM) to ablate substrate surface in near-field region. Related technologies can be found in literature[1-2].

Our technology is able to fabricate sub-wavelength features in ambient condition with nanometer precision. We use 150 femto-second pulses at wavelength of 387- nm. There are two essential advances. To improve sub- wavelength machining, we use a small probe with 150- nm apex to deliver the laser pulses. The transmitted laser beam size and beam cross section are determined by the geometry of the nanometer probe itself. Therefore, under such condition, the diffraction limit is no longer applicable, and the laser beam size can be smaller than half of the wavelength. In fact, Ohtsu et al had shown possible resolution of λ/8 [7]. The distance between fiber probe apex and the substrate surface is controlled to be much less than wavelength of 387 nm. Otherwise, laser beam would resume its free space characteristics after it propagates longer distance. To be able to precisely position the probe at the targeted location, we use the atomic force microscopy (AFM) feature of NSOM to track the sub-diffraction limit preformed features on the substrate. This strategy allows us to measure the local topology and deliver the ultra-fast laser pulses using the same fiber probe. This AFM function also enables us to control the distance between the probe and substrate very precisely. The ability of monitoring surface topology of substrate in real time enables us to accomplish the in-situ surface processing. In other words, we are able to do nanometer scale repair/adjustment on a preformed nanometer structure, which is difficult or impossible by other technique, for example etching.

   2. Technology Description: A modified operation of using NSOM (near-field scanning optical microscope) provides an ideal system for our objectives. Figure 1 illustrates the system setup. The NSOM system mainly consists of a z-motor for gross vertical movement, precision x-y-z PZT stages for nanometer motion, a tracking diode laser and a quadrant detector(not shown). The probe is drawn from an optical fiber. The quadrant detector senses the reflection of the tracking beam off the back of probe. During approaching phase, the z-motor will be disengaged when the quadrant detector detects movement of reflection due to bending of the probe. At this time, the precision PZT stage will be engaged and adjusted to maintain a constant probe bending as well as the dislocation of reflection. As a result, the force as well as the distance between the probe and the substrate surface can be kept constant. The supply voltage of z-PZT actuator can be monitored to

deduce the surface topology when the x-y PZT scans while the probe-substrate f...