Nanofabrication and its Application in Atomic Force Microscopy (AFM)
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This thesis is focused on nanofabrication and its application in atomic force microscopy (AFM). The contribution of this thesis is thus the development, investigation and characterization of novel nanofabrication technique (Part I); and application of nanofabrication in manufacturing the high aspect ratio AFM tips (Part II). In the first part of the thesis, firstly, unlike optical and mechanical lithography such as nanoimprint lithography, the throughput of EBL is very low, which demands for highly sensitive resists. We studied the dependency of e-beam exposure properties on molecular weight of the negative EBL resist polystyrene, and very high sensitivity of 1 µC/cm2 was obtained for 900 kg/mol when exposed with electron beam of 2 keV. We also demonstrated that the exposure property of high PDI (polydispersity index) polystyrene resembles that of a monodisperse (PDI 1.06) polystyrene with similar number averaged molecular weight ("Mn" ) ̅, which indicates that it is ("Mn" ) ̅ rather than ("Mw" ) ̅ (weight averaged molecular weight) that dominates the exposure properties of polystyrene resist. Secondly, lift-off using negative resist is very challenging because the resist profile is typically positively tapered due to electron forward scattering, and upon exposure negative resist is cross-linked and thus insoluble in solvents. Here we demonstrated that low energy exposure could circumvent both issues simultaneously, and we achieved liftoff of Cr with polystyrene resist using a solvent xylene. Lastly, since low energy electrons are mostly stopped inside the resist layer, radiation damage to the sub-layer is greatly reduced. Thirdly, an electron beam resist is usually coated by conventional coating methods such as spin-coating, but this cannot be reliably applied on irregular surfaces. We here reported a monolayer resist can be grafted on nonflat surface. As a proof of concept of patterning on irregular surfaces, we chose PMMA mono-layer "brush" and grafted it on irregular surfaces by thermal treatment which accelerates a chemical reaction between PMMA molecules and hydroxyl group on substrate. We achieved nanofabrication of 30 nm resolution on an AFM cantilever. Fourthly, due to the lack of feedback, conventional electron beam lithography (EBL) is a “blind” open-loop process where the exposed pattern is examined only after ex-situ resist development, which is too late for any improvement. We reported that self-developing resist nitrocellulose, for which pattern shows up right after exposure without ex-situ development, can be used as in-situ feedback on the e-beam distortion and enlargement. Once the beam was optimized using nitrocellulose resist, under the same optimal beam condition, we exposed in the common resist PMMA. We achieved ~80 nm resolution across the entire large writing field of 1 mm2, as compared to 210 nm without the beam optimization process. We also reported that self-developing resist can provide in-situ feedback for writing field alignment accuracy, which in turn can be used to optimize the alignment. In the second part of the thesis, we demonstrated the batch fabrication of high aspect ratio (HAR) AFM tips. In order to obtain high quality and faithful images in AFM, very high aspect ratio tips are required in order to reach to the bottom of narrow and deep trenches/holes. But these HAR tips are extremely difficult to make and consequently very expensive. Currently all the commercially available HAR AFM tips are fabricated in a slow, costly (~5-20 that of regular AFM tips) and serial manner (one by one). We here developed a method to batch fabricate HAR AFM tips by forming a hard metal etching mask just on the apex of the pyramid tip followed by silicon dry etching to achieve the HAR pillar right below the metal island mask. Since it is a batch and lithography-free process, it has much higher throughput and much lower manufacturing cost per tip. This technique was first successfully applied on large-area pyramid arrays and then transferred to the commercial regular AFM tips, and has demonstrated the uniformity, reproducibility and yield of those HAR tips. The tip apex diameter and tip pillar height are controllable by tuning metal thickness and silicon dry etching time respectively. Finally, we demonstrated that the HAR tips fabricated using our technique gave a better imaging quality than the commercial regular tips.