Saturday, September 8, 2018

Combining Focused Ion Beam Patterning and Atomic Layer Deposition for Nanofabrication

While the big guys are banging there heads against the wall to achieve smaller critical dimensions for Logic and Memory fabrication using classical top down Litho-Etch patterning ALD has stepped in multiple times to save the world. Some examples in high volume manufacturing:
  • ASD - Area selective deposition by ether CVD or ALD
  • SADP - Self-aligned double patterning
  • SAQP - Self-alignes quadruple patterning
  • Depositing hardmask materials and liners in advanced patterning schemes for high aspect ratio and dense features
Besides ALD, ALE is used for trimming of pattern features such as FinFETs and hardmasks, or to fabricate smallest feature nano-imprint stamps and even to split nano wires longitudinal. There is basically no end to what you can do once you have atomic level control of things.

Focused Ion Beam (FIB) is a technology that is available in most material analytical labs and fabs as in-line metrology to make sample preparations or repair and is also used in standard manufacturing for lithography masks, i.e., there are tools out there that can shuffle substrates and 300 mm wafers at high speed.

Ph.D. Thesis defence (picture from Twitter, HelsinkiALD

That is why this thesis by Zhongmei Han is indeed very interesting and worth reading - congratulations to the recent defense in Helsinki!

Combining Focused Ion Beam Patterning and Atomic Layer Deposition for Nanofabrication

Doctoral Thesis, Zhongmei Han
Department of Chemistry, Faculty of Science, University of Helsinki, Finland

For nanofabrication of silicon based structures, focused ion beam (FIB) milling is a top-down approach mainly used for prototyping sub-micron devices, while atomic layer deposition (ALD) is a bottom-up approach for depositing functional thin films with excellent conformality and a nanometer level accuracy in controlling film thicknesses. Combining the strengths of FIB milling with ALD provides new opportunities for making 3D nanostructures. In FIB milled silicon, the gallium implanted surface suffers from segregation and roughening upon heating, which makes the thermal stability of the as-milled substrate a concern for the following ALD processes which are typically performed at temperatures of 150 ℃ and higher. This study aimed to explore methods for improving the thermal stability of FIB milled silicon structures for the following ALD processes. The other aim was to fabricate nanostructures by alternately using FIB milling and ALD approaches on silicon and oxide thin film materials. The experiments were started on the reduction of gallium implantation during FIB milling of silicon substrates using different incident angles. Oblique incidence of the ion beam was found an effective method for improving the thermal stability of the FIB milled silicon surfaces by decreasing their gallium content. The improved thermal stability allowed to apply ALD Al2O3 on the FIB milled surfaces to make nanotrenches. Wet etching in KOH/H2O2 was found as a second method for improving the thermal stability by removing the gallium implanted silicon layer. ALD Al2O3 thin films can be applied as milling masks to limit amorphization of silicon upon FIB milling. With the aid of KOH/H2O2 etching, nanopore arrays, nanotrenches and nanochannels were fabricated. ALD grown Al2O3/Ta2O5/Al2O3 multilayers were FIB milled and wet etched to form both 2D and 3D hard masks. The fabricated 2D masks were used for making metal structures which are applicable for electrical connections. Thin film resistors were also fabricated using this 2D mask system. In conclusion, this study illustrates that combining FIB patterning and ALD is feasible for 3D nanofabrication when the stability of FIB milled surfaces is considered and improved. 

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