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Plasma Activated Bonding for an Enhanced Alignment Electrostatic Lens
Keywords: Electrospray beam Focusing, Electrostatic Lens, Plasma Activated Bonding
The energetic impact of electrostatically accelerated beams of electrospray nanodroplets can be used for material deposition, sputtering, and milling processes; all of which are currently done by beams of atomic and cluster ions [2], [4]. The hypervelocity impact by electrosprayed nanodroplets is a promising tool in microfabrication technology and surface engineering; however, to offer improve capabilities, higher image resolution required. Higher sputtering yield has been shown in the energetic impact of the ES nanodroplets due to higher kinetic energy which is controlled by the accelerating voltage and charge to mass ratio of the spray of ionic liquid [1]. Up to 25 times improvement in the etching rate of ES beam over ion beam was reported in patterning a Si substrate [3]. This paper presents design and microfabrication of an electrostatic lens as a part of a focused electrospray beam apparatus for high resolution surface scanning and milling. The high energy density of the focused beam is desired for ejecting atoms from target's surface for high rate etching and sputtering purposes. An axisymmetric Einzel lens including three electrodes is proposed to focus the coaxial electrospray beam in our experimental setup. In our design, the post fabrication assembly is eliminated when silicon electrodes and glass spacers are permanently bonded using plasma activated wafer bonding. Minimizing fabrication errors and electrodes misalignment are essential in order to minimize the geometrical aberration sources such as astigmatism. Glass wafers are used as the dielectric spacers between silicon electrodes to provide the uniform gap. To have the best alignment between lens electrodes and avoid detachment in further processing, a permanent direct bonding is preferred over adhesive bonding. Plasma activated bonding was chosen due to simplicity and lower processing cost. First a stack of bonded silicon and glass wafers are fabricated and Si electrodes are etched. Next, the third electrode and glass wafers are bonded to the etched electrode stack. When the third electrode is DRI etched, glass wet etch opens the holes through the stack of lens unit. Si wafers also play the role of hard mask for glass etching. The advantage of our approach over etching all electrodes in one step [5],[6], is minimum aperture size mismatch. i.e. the top electrode is not over etched and the bottom electrode is not under etched. An optical observation verifies the alignment between the axis of central apertures. The glass standoffs in the lens unit proved to be able to provide a breakdown voltage of up to 22kV in vacuum. This threshold is higher than potential needed for focusing the accelerated ES beam.
Elham Vakil Asadollahei, PhD Candidate
University of California, Irvine
Irvine, CA

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