NPGS : Dots in PMMA
shows the entire pattern when it is exposed with a dose that fully clears
The images below show parts of the same pattern when a low dose is used with a field emission SEM. The center-to-center spacing of the exposure points is only 50 nm, which means that the dots size is on the order of 15 nm or smaller.
Because the resist was ~200 nm thick, the dots shown below may not be developed completely through the resist, however, the size and regular placement of the dots shows that the SEM had very little external noise affecting the beam. When there is external noise, the dots will not be as uniform and evenly placed as shown below. Consequently, exposing a pattern such as this one at a low dose provides a very good diagnostic test of the beam optimization and the noise environment of the microscope.
Note that a field emission microscope will typically be able to produce isolated dots on a 50 nm x 50 nm grid, while a W or LaB6 microscope will rarely be able to make such small, closely spaced features.
image shows a close view of the upper left corner of the serpentine structure
when the exposure has produced dots on a 50 nm square grid.
The 20 rows and 20 columns of dots can be counted that make the 1 um width of the serpentine structure.
This sample has been developed and sputter coated with gold. The diameter of these dots is on the order of 15 nm.
The dot exposures were made at the Central Analytical Facility at The University of Alabama.
image shows the very top of the fractal star on the right.
Here, it can be seen that the exposure routine of NPGS does not use a simplistic XY raster when filling arbitrary polygons. Instead, NPGS makes parallel passes of the beam where the point spacing along each pass is optimized to best match the length of the pass, while fully utilizing the available 16 bit resolution of the DACs. In this case, the beam made horizontal passes that were separated by 50 nm and along each pass the dots were separated by 50 nm. Since the sides of the star are sloped, each horizontal pass has been shifted to best fit the specified length.
Note that with NPGS the passes could also have been parallel to any side of the fractal star.
image shows the lowest part of the ring structure. The interesting part of
this image is that the dots to the right of center were exposed first
with the beam moving up and down as it stepped in from the right. After the
bottom right part of the ring was exposed, the beam moved to expose the rest
of the ring structure, while always keeping the beam moving parallel to the
inital sweep direction. The final part of the ring to be exposed was the
area to the left of center in this image. Note how no discontinuity can be
seen were the two scan areas meet.
This ring structure and horizontal connecting leads was designed as a single filled polygon with short straight segments defining the apparent curve of the ring. With NPGS, the ring could also have been exposed as a wide circle. In that case, NPGS would have made concentric passes of the beam starting from the outside edge moving towards the center.