3D FIB nanolithography using nanomachining technics
The Focused Ion Beam (FIB)microscopes are extermely versatile and powerful instruments for materials research.
It’s a tool to realize sample imaging, sectioning, ion-lithography and nano/micro machining on surfaces. The art to produce complex shapes at the nanoscales is often limited with the default pattern engine included in the instrument software or you need to buy a very expensive add-in for enlarging the potentiality of your pattern strategy.
An alternative way to improve the complexity of your pattern is the ‘stream file’. They are text files with tens of thousands of rows where each row contains the coordinates of the ion beam with the dwell time. The quality of the pattern geometry is influenced by an huge number of phisical and process parameters: Ion beam voltage, current and diameter; sample material (roughness, cristallinity, electrical conducibility, melting points, etc); scan strategy, pitch overlap, etc.
The stream file is a powerful method to create a fully customizable milling strategy. The implementation of a software interface, with few input parameters allows to compile a stream file in 2-3 minutes without advanced user competence.
The flowchart below shows the method that starts from a 3D CAD model or a bitmap where each grey value is coupled with the dwell time. Then, the feature is patterned using ion lithography and Indentation lithography (for large areas).
- Pattern 3D CAD design where the Z represents the dwell time and discretization using triangular mesh (rectangular mesh often generates problems)
- An home-made VB.net interface permits to set ion parameters, dimensions, scan strategy, material (Sputter Yield) and to translate the node coordinates in a stream file
- A stream file contains X and Y coordinates with the relative dwell time
- Pattern milling using a FEG-FIB dualbeam instrument
- Script routine based on DIC technique developed to reduce beam drift and repeat the pattern on large areas (Ion Lithography)
- According the total area to be milled and material, the ion lithography could be performed by Direct Writing (small areas) or through the nano-fabrication of an head-print and Indentation Lithography with a Nanoindenter with ultra-fast capability (1 indentation/s), if pattern on large area is required.
Head-print fabrication for indentation lithograohy was designed developing a new ion beam scan method on the sample surface that we call: Boundaries Pursuing Strategy.
After the model nodes transformation, a matrix is available as a function of current and requested geometry, where each pixel is defined with the relative dwell time. In the present article, a new scan strategy is developed not only to improve the quality of the milled pattern but also to produce high aspect ratio patterns with sharp edges and low re-deposition. In order to achieve this, an algorithm has been produced based on the “neighboring pixels check” which allows to sort the stream file rows in an appropriate sequence that is followed by the DAC scan generator. The scan strategy is explained in the next image to mill a simple circular pillar (that means remove material around it) and it involves the following steps:
1/ A ‘checking matrix’ constituted by eight neighboring pixels filled with the relative dwell time values has been adopted, where the center is the beam current position as shown in the magnified view. Starting from the upper left corner of the scan pixel map (point 1), each position is evaluated by the eight surrounding values of checking matrix.
2/ Starting from the right side of the checking matrix, the position will be listed in the stream file only if it contains a dwell time higher than zero, otherwise the code continues clockwise movement checking the neighboring pixel. When the code finds a value greater than zero, the beam is moved to the relative scan pixel position and consequently the checking matrix is moved and centered on the new location and so on. On each movement step, the dwell time of the previous position is converted to zero to avoid that the beam crosses that pixel again.
The procedure is completed once the entire map of pixels is scanned (that means all the values are zero). With this logic, the ion beam moves pixel by pixel (clockwise) form point 1 to point 2 then it meets the ‘no milling area’ and starts to follow the pillar boundaries (up to point 3) and finally it keeps going, up to the end. In other words, the ion beam doesn’t change its path while it meets pixels with null value. Depending on the geometry, the checking matrix can be scanned clockwise or counter-clockwise. In this way redeposition is decreased and pattern edges are preserved because the blanking operations are strongly reduced.
In the following video is shown a simulation of the ion beam movement using the Boundaries Pursuing Strategy to realize three different pillars
The method can be applied to nanofabricate a master for nano-indentation lithography as shown in the image below:
Another interesting result obtained with the developed procedure is the fabbrication of another diamond tip to usa as a master containing both nano and micro features. In particular a matrix of truncated cones with a nano Fresnel lens patterned on the top was milled as shown below.
The mentioned master has been tested on different substrates to evaluate the pattern produced. The best results have obtained on a bulk metal glass (BMG) (see the following images)